Random Access Memory (RAM) http://www.edbpriser.dk/review.asp?ID=1586 Random Access Memory (RAM) er computerens såkaldte midlertidige datalager, hvor operativ systemet (eksempelvis Windows), applikationer (programmer, spil, m.m.) og data, der på et givent tidspunkt er i brug, bliver gemt. Når du eksempelvis starter et program på computeren, er det her der indlæses nødvendigt data, som kræves for at applikationen kan afvikles. Mens en applikation er installeret på harddisken og tilgængelig derfra, er den ikke brugbar før den er indlæst i denne midlertidige hukommelse - RAM. Når du oplever at computeren begynder at arbejde, efter du har startet et program, spil, m.m., er det er fordi den er i gang med at overføre de nødvendige data til computerens RAM. Det er computerens CPU (Central Processing Unit), der udfører de individuelle instruktioner, som applikationen forespørger. Billedet nedenfor illustrerer i grove træk denne sammenhæng. Et godt eksempel er når computeren starter et program, såsom dit E-mail program. Processen der går forud for at programmet er tilgængeligt på skærmen for dig som bruger, er at computerens CPU modtager en instruks fra dig ved at du starter programmet, normalt ved et klik med musen. Her oversætter CPU’en denne instruks, og sender en besked til harddisken om at de pågældende program skal indlæses i hukommelsen. Nu ligger programmet i computerens hukommelse, altså rammen, og klar til at bruge. Jeg vil senere forklare hvorfor det ekstra led, som rammen utvivlsomt er, er nødvendigt, men jeg kan allerede nu afsløre at det hele handler om hastighed. Hvis man skal forsøge at perspektivere hvad RAM er for noget, der måske er mere håndgribeligt for folk der ikke lige kender til alle de ovennævnte begreber, kan det sammenlignes lidt med menneskets korttidshukommelse. Det du lige er i gang med ligger i korttidshukommelsen (Rammen), og kan du ikke lige huske hvad du skal gøre, trækker du på langtidshukommelsen (Harddisken). Det samme gør sig gældende for computeren. Hvis rammen er fyldt op eller de nødvendige data ikke er tilgængelige, hentes der yderligere data fra harddisken ind i rammen. RAM er som benævnt ovenfor en del af en samlet computer (PC). RAM kan altså ikke fungere alene, og en computer kan ikke benyttes uden RAM. Det sidste er ikke helt rigtigt, da alle computere i sig selv er udstyret med et minimum af RAM. Dette minimum er dog yderst begrænset, og vi skal mange år tilbage før at en computer reelt kunne fungere uden ekstra RAM klodser. Dagens standard af programmer og spil kræver stadig mere og mere RAM, og derfor ville det i dag være usandsynligt at have en computer uden. Det har som sagt ikke altid været tilfældet, og før de første såkaldte Personlige Computere (PC) kom på markedet i 1980’erne havde Bill Gates udtalt følgende:”640K of memory ought to be enough for anybody”. Det skulle hurtigt vise sig at være forkert, og i dag er tendensen faktisk at uanset hvor meget hukommelse, der installeres i en computer, vil det altid være bedre med mere. Hvordan virker RAM: DRAM kan tænkes som værende en stor tabel med en række celler opdelt i kolonner og rækker. Cellerne består af capacitors og indeholder en eller flere bits data, afhængig af den specifikke chipkonfiguration. Tabellen tilgås via dets rækker og kolonner der på skift modtager signaler fra RAS (Row Access Strobe) og CAS (Column Address Strobe). Capacitorerne, der bruges i cellerne til at gemme data, aflades efter et bestemt stykke tid, og skal derfor opdateres periodisk for at data ikke går tabt. En opdateringskontroller bestemmer intervallet mellem hver opdatering, mens en opdateringstæller sørger for at alle rækker bliver opdateret korrekt. Det der sker når CPU’en skal bruge information fra hukommelsen er at denne sender en forespørgsel, der så håndteres af Hukommelseskontrolleren. Hukommelseskontrolleren sender så forespørgslen videre til selve hukommelsen, og rapporterer tilbage til CPU’en når information er fundet og klar til brug. Hvor lang tid denne cyklus tager, fra CPU’en til hukommelsescontrolleren til selve hukommelsen og tilbage igen, variere i forhold til rammens hastighed, systemets bushastighed, samt en række andre faktorer. De vigtigste faktorer, der spiller ind på et moduls hastighed, er: Hastigheden (MHz) Burst timings Data Bus Bredde Samlede båndbredde Disse faktorer er udførligt forklaret gennem denne guide til RAM. I sidste ting er tilgangen til hukommelsen, der enten fungerer gennem en synkron eller en asynkron interface. Den synkrone interface betyder i virkeligheden en langsommere hukommelse, eftersom RAM modulet kun kan udføre en intern operation ad gangen, mens den asynkrone kan udføre flere operationer samtidig. Forskellen på RAM og Harddiske: Der er flere markante forskelle mellem RAM og harddisken. Først og fremmest er RAM, som tidligere nævnt, kun en midlertidig hukommelse, der slettes når computeren slukkes. Dernæst er RAM betydelig hurtigere, og netop hovedårsagen til at den er nødvendig. I dette afsnit vil jeg starte med at forklare de begrebsmæssige forskelle, hvor de tekniske vil blive gennemgået senere i guiden. Forestil dig en metafor, hvor du har et skrivebord og filekabinet på et kontor. En kunde ringer og spørger til sin sag. Du rejser dig, åbner filekabinet og finder den pågældende kundes sag. Du snakker med kunden og noterer samtalens højdepunkter, hvorefter du arkiverer sagen igen. Denne situation kan man med stor fordel overføres til sammenhængen mellem rammen og harddisken i en computer. Dit skrivebord er rammen, når du har sager liggende der er de tilgængelige, og du kan hurtigt slå op i dem. Havde du i eksemplet allerede haft kundens sag på skrivebordet, var du ikke nødsaget til at rejse og gennemlede filekabinet for at finde oplysningerne frem. Det er åbenlyst at sager der ligger på skrivebordet er meget hurtigere at tilgå end dem i filekabinettet. På samme måde er det i en computer, men hvor stor er hastighedsforskellen så? For det første måler man harddiskes hastighed i millisekunder, mens ram måles i nanosekunder. En anden ting er den tid det tager for CPU’en at tilgå henholdsvis rammen og harddisken. For rammen er det ca. 200ns, mens det for harddisken er 12.000.000ns, hvilket vil sige at rammen her, er godt 60.000 gange hurtigere. For at sætte det hele lidt i perspektiv viser illustrationen nedenfor dette omsat til minutter. Hvis du nu tænker at harddisken er noget langsomt skidt, som der ingen grund er til at investere i, eller som forskerne i hvert fald burde udvikle lidt mere, må du huske den meget væsentlige forskel i at ram er midlertidigt lager mens harddisken er vedvarende. Hvor meget hukommelse er nødvendigt? (Lagerkapacitet): Måske kender du allerede til at arbejde på en computer hvor der ikke helt er nok hukommelse. Du hører ofte harddisken operere, hele computeren virker sløv og nogen gange kan du ikke få lov til at starte et nyt program uden først at lukke et andet. Hvordan ved du så om mere hukommelse kan afhjælpe problemet? Mængden af hukommelse, som du skal bruge eller har behov for, afhænger primært af to ting. Først og fremmest afhænger det af hvad du bruger din computer til – hvor krævende og hvor mange applikationer du anvender. Dernæst har det styresystem du benytter også indvirkning på hvor meget ram du bør installere. Nedenstående skal forstås som retningslinier for hvor meget hukommelse din computer bør have, baseret på styresystemet og de applikationer du anvender. Windows 9x (Windows 95 / Windows 98 / Windows ME): Windows 9x kræver minimum 32MB RAM, mens det optimale ide flestes tilfælde er 128MB eller mere. Ser vi på Windows Xp kræves der som minimum 128 MB RAM, mens computere i dag ofte sælges med mindst 256 MB, dog vil 512 MB RAM være anbefalelsesværdigt. Bruger du primært din computer til tekstbehandling, regneark, E-mail og en smule Internet surfing kan de anbefales at have mellem 128 og 256MB RAM. Hvis du har 128MB vil du kunne anvende programmerne nogenlunde, men vil nogen gange mærke at computeren løber tør og der vil være heftig aktivitet på harddisken. Har du derimod 256MB vil det hele køre mere let, og det vil kun være hvis du har mange programmer åbne at du vil opleve at computeren får svært ved at følge med. Bruger du din computer til hele kontorprogrammer (eksempelvis Microsoft Office), spil, en del Internet Surfing, billeder og præsentationer skal du have mere RAM. Her anbefales som minimum 256MB, men helst 512MB. Sætter du virkelig din computer på arbejde, og bruger den til multimedia med billeder, video og lyd sammen med andre opgaver som Internet, E-mail, tekstbehandling, m.m. skal du op og have meget RAM før computeren kan håndtere dine behov. Her er et absolut minimum 512MB, men generelt jo mere jo bedre. *Værd opmærksom på at Windows 9x ikke er optimeret til at udnytte store mængder RAM, og du vil derfor ikke få det optimale ud af at installere mere end 128MB. Hvad gør man så hvis man har det sidst nævnte behov og eksempelvis gerne vil installere 256MB? Ja enten gør man det og acceptere at styresystemet ikke udnytter det 100% optimalt, eller også opgradere man sit styresystem. Windows 2000 Professional / Windows XP Home/Professional: Windows 2000 og Windows XP er meget ens styresystemer, baseret på det samme fundament, hvor XP sådan set bare er en finpudset version af 2000. De kræver begge som absolut minimum 64MB RAM, for at kunne kører, men det kan ikke anbefales at have mindre end 128MB, og min erfaring er endvidere at de ikke kører optimalt før man runder 256MB. Begge styresystemer har en 4GB grænse for RAM. Benytter man Windows XP’s 64-bit version kan man dog installere op til 16GB. Bruger man sin computer til mindre krævende opgaver, som tekstbehandling, E-mail osv. kan man i teorien klare sig med 64MB RAM, mens jeg dog i praksis har erfaret at 128MB er en god investering. Stiller man større krav til de opgaver ens computer skal kunne håndtere skal man nok op og have 256MB. Sådanne opgaver kunne være Internet surfing med flere browsere åbne på samme tid, komplekse præsentationer, billedbehandling, database administrering, mindre krævende software udvikling (eksempelvis web-udvikling), osv. Selv til disse opgaver kan det være en hastighedsmæssig fordel at installere 384 eller 512MB RAM. Forventer man en computer der skal kunne håndtere statistik programmer, real-time video, animation, storstillet Internet Surfing, heftig netværks trafik, osv., bør man installere 256MB som minimum og gerne helt op imod 1GB RAM. Linux: Styresystemet Linux har efter eksisteret i nogle år, og har efterhånden opnået popularitet. Kravene til Linux hvad angår RAM er ikke meget anderledes end hvad de nyere Windows systemer kræver. Hvor meget RAM Linux maksimalt understøtter afhænger a hvilken version og hvilket mærke (RedHat, SuSe, osv.) man har installeret. Som udgangspunkt understøtter de nyeste versioner 4GB RAM, mens specielle serverversioner kan understøtte helt op til 64GB. Hvor meget RAM skal man så installere? Jo bruger man sin computer til eksempelvis tekstbehandling, E-mail, simpel billedmanipulation kan man klarer sig med 48-80MB. Ligesom ved Windows er den nederste værdi i intervallet Linux eget vurdering af hvad minimumskravet er. Er dine behov en smule mere krævende, og bruger du din computer til multimedia præsentationer, databaser, Internet Surfing, osv. bør du som minimum have 80MB men gerne mere. Er vi helt ude hvor dine behov virkelig er krævende, og hvor du benytter statistik programmer, video samtaler, kompleks billedbehandling, animation, m.v. bør du have 112-512MB. Macintosh: Macintosh håndterer hukommelse på en væsentlig anderledes måde end andre styresystemer. Jeg vil ikke gå i deltager med dette, men konsekvensen er at Macintosh computere kræver mere RAM. 48MB er et absolut minimum, men mindre end 128MB kan ikke anbefales. Igen afhænger behovet for RAM af hvad du bruger din computer til. Tekstbehandling, E-mail og den slags mindre krævende applikationer vil kræve omkring 64MB RAM, mens tungere applikationer kan kræve fra 128MB og opefter. * Her opmærksom på at ovenstående vurderinger af den nødvendige mængde af RAM er baseret på et typisk brug af computeren. Eksempelvis vil antallet af applikationer man har åbnet samtidig også ave en betydning for hvor meget RAM man bør installere. Som tidligere nævnt er det heller ikke helt forkert at sige at jo mere RAM jo bedre, uanset hvad man benytter computeren til. Det kan også godt betale sig at investere i mere hukommelse end man umiddelbart har brug for, sådan at man bare er en smule fremtidssikret, da opgraderinger i software mange gange kræver højere minimumskrav. Opgradering af computerens RAM: Har du en lidt ældre computer, og synes du efterhånden at den har svært ved at følge med kan det mange gange godt betale sig at opgradere den i stedet for at kassere den og købe en ny. Ofte vil en opgradering af mængden af RAM bevirke en betydelig hastighedsforøgelse. Hvis du beslutter dig for at installere mere RAM i din computer er der en række ting du skal være opmærksom på. Først og fremmest skal du undersøge hvorvidt der er nogen frie RAM slots i computeren. Billedet nedenfor viser en skitse et bundkort, hvor RAM slots er markeret. Er der ikke nogen frie RAM slots, er man nød til at fjerne en RAM klods og installere en med større kapacitet hvis man ønsker at opgradere. Udover at undersøge om der er plads på bundkortet, skal man også undersøge hvilken type RAM ens bundkort understøtter (mere om det senere). Der er både forskellige typer af RAM og forskellige hastigheder, der begge kan være afgørende for om en bestemt RAM klods passer til netop dit bundkort. Du skal have altså have fat i manualen til dit bundkort, eller finde ud af hvilket bundkort det er du har, og via Internettet finde frem til manualen på eksempelvis producentens hjemmeside. Kan du ikke finde frem til den, har du også muligheden for at tage en af de allerede installerede RAM klodser ud, vise den til en forhandler og på den måde købe sammen type, bare med en større kapacitet. Køling: Førhen var der ikke noget der hed køling når det kom til RAM moduler, men efterhånden som teknologien indenfor RAM har udviklet sig, og modulernes kapacitet er bleget større og større samtidig med forøget hastigheder, er varmudviklingen også steget. Det er blevet nødvendig at nedkøle RAM chippen. Det kan altså godt anbefales at se efter RAM klodser der har fået spændt nogle kobber pladder udenpå sig, for at nedkøle RAM klodsen. Dette forlænger teoretisk set rammens levetid, og giver mulighed for højere RAM hastigheder. Specifikationer på EDBpriser: Følgende er de specifikationer som er de mest interessante og dem der kan have en betydning for valget RAM modul. Port Type (Model): Afhængigt af hvilket bundkort du har i din computer skal du også have RAM, der passer til de porte, som findes på bundkortet. For den uerfarne bruger kan dette godt virke som noget af en videnskab, selvom det faktisk er rimelig enkelt at bestemme. Problemet er så bare at flere forskellige RAM typer fysisk passer til de samme porte, og det er derfor ikke nok at RAM klodserne passer. Her er man nød til have fat i manualen til bundkortet for at være sikker (mere om de forskellige RAM typer senere i guiden). Følgende er en kort gennemgang af de forskellige slot typer der findes. Single In-Line Memory Module (SIMM): De første SIMM moduler kunne overføre data med 8 bits ad gangen og havde 30 ben, derfor blev de kaldt 30-Pin SIMM’s. Senere udviklede man en forbedret version med 72 ben, der kunne overføre data med 32 bit – 72-pin SIMM’s. SIMM moduler bliver næsten ikke brugt mere. Dual In-line Memory Modules (DIMM): DIMM porte er det man ser på langt de fleste bundkort i dag. Der er to varianter, en 168-pin og en 184-pin. Den primære forskel på henholdsvis SIMM og DIMM modulerne er: Benene på hver side af et SIMM RAM modul er sammenkædet så de danner en fælles elektrisk kontakt til SIMM porten på bundkortet, mens DIMM moduler danner to separate kontakter. De mere fysiske forskelle er bl.a. længden, hvilket siger sig selv når man tager antallet af ben i betragtning. Der er også en mindre forskel i måden man installere modulerne på. SIMM modulet installeres en anelse skråt, mens DIMM modulet installeres fuldstændig vertikalt. Small Outline Dual In-line Memory Modules (SO-DIMM): Denne porttype bruges i forbindelse med bærbare computere. Bærbare computere er mere komprimeret, og har derfor også brug for komponenter der er mindre. Dette tilbyder SO-DIMM RAM moduler. Den eneste principielle forskel på SO-DIMM og DIMM, er at SO-DIMM er markant mindre, da den jo netop er tiltænkt bærbare computere. SO-DIMM moduler kan have henholdsvis 72, 144 eller 200 ben, og heraf navnene 72-PIN SO- DIMM der er 32bit, 144-PIN SO-DIMM der er 64Bit og 200-PIN SO-DIMM der er 72bit. Rambus In-line Memory Modules (RIMM): RIMM ligner DIMM til forveksling, dog værende en anelse tykkere. Den hurtigere overførselshastighed (se senere) medføre en højere varmeudvikling, og derfor er et RIMM modul dækket af et aluminiumdæksel, en såkaldt heat spreader, for beskyttelse mod overophedning. Small Outline Rambus In-line Memory Modules (SO-RIMM): SO-RIMM er det samme som SO-DIMM, bortset fra den bruger Rambus teknologien. Hukommelsestyper (Memory): Du behøver ikke bekymre dig sig meget om hvilke porte netop dit bundkort benytter. Medmindre din computer er meget gammel, altså før 1997, er det næsten med 100% sikkerhed DIMM porte. Du skal bare lige kigge din manual til bundkortet igennem, og checke hvilken porttype det understøtter. Har du ikke din manual kan du også gennem windows kontrol panel undersøge hvilket bundkort du har, og så via producentens hjemmeside porttypen derigennem. Det som du skal være opmærksom på er hvilken RAM type du skal have fat i. Der findes mange forskellige typer RAM, og som nævnt i forrige afsnit ligner nogle af dem til forveksling hinanden. Herunder følger en gennemgang af de mest kendte. Asynkron Fast Page Mode (FPM) DRAM: Ved at implementere nogle specielle metoder til at tilgå data i hukommelsen, er det lykkedes at mindske de interne ventetider ved nogle typer datatilgange. Denne teknologi fik tilnavnet Fast Page fordi resultatet var at en hel ”side” af data kunne holdes aktiv i rammen. Fordelen var dog kun mærkbar ved afvikling af nogle applikationer, og teknologien blev hurtigt videreudviklet. Fast Page Mode (FPM) kom til verdenen , og blev standarden indenfor RAM fra 1987 til midten af 90'erne. Kort fortalt var fordelen ved FPM var at den simpelthen var hurtigere til at tilgå data i rammen, sammenlignet med konventionel DRAM. Hastighedsgrænsen er 50ns. FPM DRAM ses meget sjældent nu til dags. Har man alligevel behov for at benytte denne type RAM, skal man være opmærksom på at den ikke understøtter en BUS hastighed på højere end 66Mhz. FPM DRAM vil typisk kunne anvende Burst Timings på 5-3-3-3 ved 66MHz. Passer i en RIMM port, og har 72 pins. Asynkron Extended Data Out (EDO) DRAM: EDO RAM så dagens lys i 1995, og udkonkurrerede langsomt FPM rammen. EDO RAM er en anelse hurtigere en FPM DRAM pga. en nyere og revolutionerende ændring i måden hukommelsen tilgås. Kort fortalt handler det om at EDO RAM kan påbegynde en ny tilgang til rammen før den forrige tilgang er færdigbehandlet. Den kan også både læse og skrive til rammen samtidig, og på den måde springe nogle trin over i processen af adressering af hukommelse. Ligesom FPM er denne ramtype næsten uddød, og bliver næsten ikke produceret længere. EDO RAM kan opnå hastigheder på helt op til 100Mhz, hvis altså bundkortet understøtter det. Problemet her er netop den tids bundkort, der sjældent havde en bus hastighed på højere end 66Mhz, og derfor er fordelen, i forhold til FPM RAM, reelt ikke særlig stor på dette område. Hvad angår Burst Timings er der noget at hente. EDO RAM understøtter Burst Timings ned til 5-2-2-2. Efter sigende skulle rammens tilgangshastigheder også være 40% hurtigere end FPM RAM. Alt Dette øger teoretisk set hastigheden med ca. 10-15%. I praksis viste det sig imidlertid et være en del mindre, faktisk helt ned til kun 1%, hvor man så må konkludere at en opgradering til EDO RAM næsten er ubetydelig. Hastighedsgrænsen er ligesom FPM DRAM 50ns. Passer i en RIMM port, og har 72 pins. Burst Extended Data Out (BEDO) DRAM: BEDO DRAM er endnu en forbedring af de konventionelle asynkrone DRAM. Forbedringen ligger i at Burst Timings er blevet presset helt ned på 4-1-1-1. Dette kræver naturligvis at bundkortet understøtter disse hastigheder. BEDO RAM giver en større hastighedsforøgelse i forhold til EDO, end hvad EDO RAM gav i forhold til FPM RAM. Trods det har det aldrig rigtigt slået igennem, hvilket primært er fordi chipgiganten Intel valgte ikke at understøtte denne RAM type. Passer ligeledes i en RIMM port, og har 72 pins. Synchronous DRAM (SDRAM): I slutningen af 1996 kom SDRAM. I modsætning til tidligere RAM teknologier, kan SDRAM synkronisere sig selv med CPU'ens klokfrekvens. Det gør at chippen på bundkortet, der styrer interaktionen mellem hukommelsen og CPU'en, kender den eksakte cyklus af klokfrekvensen, og derfor behøver CPU'en ikke længere afvente, når data i hukommelsen forespørges. En anden fordel ved SDRAM er brugen af Interleaving, der gør at et halvdelen af et RAM modul kan færdiggøre en proces mens den anden halvdel påbegynder en ny. Videre benytter SDRAM såkaldt Burst mode, der eliminerer ventetiden mellem hukommelsen og CPU’en. Både bugen af Interleaving og Burst mode øger rammens ydeevne betydeligt. Hastigheden for SDRAM startede med at være 66Mhz, men blev frem til år 2000 først øget til 100 og derefter til 133MHz. Siden hen er der ikke sket nogen forbedring. SDRAM understøtter Burst Timings på 5-1-1-1 Når det kommer til SDRAM kan det være meget forvirrende, at finde ud hvilke der lige passer til ens bundkort. Der er mange faktorer, der spiller ind for hvorvidt en bestemt SDRAM klods kan anvendes. Eksempelvis må RAM klodsernes hastighed ikke være langsommere end systembusen på bundkortet, ellers vil de ikke virke eller også vil de være meget ustabile. Det er fordi hele pointen med SDRAM teknologien netop er, at eliminere de ventetider som den konventionelle asynkrone RAM var påvirket af, og derved give en bedre ydeevne. Står du med et bundkort der understøtter SDRAM er det sikreste at kontakte producenten af bundkortet, og høre hvilke specifikationer RAM klodserne skal have. Passer i en DIMM port, og har 168 pins. Direct Rambus DRAM (RDRAM / DRDRAM): Rambus er en DRAM arkitektur der er noget anderledes end de andre DRAM designs. Direct Rambus teknologien er på papiret ekstraordinær hurtig, sammenlignet med de andre tilgængelige teknologier. Det skyldes dets evne til at dobbelt klokke. Det betyder at den er i stand til at udnytte både den stigende og den faldende klokfrekvens og udfører operationer i begge henseender. Netop dette gjorde at Rambus teknologien var meget revolutionerende, og i sin tid blev spået som fremtidens førende RAM teknologi. Det er dog ikke blevet tilfældet, måske pga. Rambus og Intels patenter. Hvis en chipfabrikant havde intentioner om at udvikle RAM moduler baseret på denne teknologi, skulle de betale afgifter til Intel og Rambus. Dette er ikke særlig attraktivt og gør det svært for en chipfabrikant at konkurrere. Samtidig ville chipfabrikanten være underlagt de standarder som Intel og Rambus bestemmer, og derved ikke have nogle egentlig kontrol med udviklingen. Rambus har en hastighed på op til 800MHz, og en teoretisk båndbredde på 1.6GB per sekund. Trods en væsentlig øget frekvens er den samlede båndbredde for RAMBUS dog kun dobbelt så stor som for 100MHz SDRAM. En sidste ting der gør RDRAM unik, er busen Direct Rambus Channel. Andre RAM typer benytter en lignende bus, men hvor de søger at øge båndbredden af denne, for at kunne overføre mere data ad gangen, har Rambus mindskes, for at overføre data hurtigere. Normalt benyttes en bus på 64 bits hvor RDRAM kun benytter 16 bit. Det betyder at chippens hastighed på 800Mhz i praksis kun yder 400Mhz sammenlignet med Ram der benytter en bus på 64 bit. Hvad angår Burst Timings er RDRAM ikke noget at råbe hurra for, og disse hastigheder er faktisk værre en SDRAM. Netop dette er alvorligt fordi mange programmer ikke er gearet til at udnytte den høje bus hastighed, og derfor i virkeligheden ville have mere gavn af hurtigere Burst Timings. Konklusionen må være at RDRAM er for dyrt sammenlignet med dets ydeevne. Samtidig er RDRAM heller ikke så udbredt som DDR SDRAM (Se næste afsnit), og det begrænser udvalget af bundkort som understøtter modulerne. Passer i en RIMM port, og har 184 pins. Synclink DRAM (SLDRAM): Selvom denne Ram type for længst er blevet erklæret uddød, synes jeg den er hver at nævne, fordi den i slutningen af 1990erne var et alternativt bud på en konkurrent til RDRAM. SLDRAM er et revolutionerende design hvis primære forhold er en forøget ydeevne ved brug af eksisterende arkitektur. Altså formålet var at opnå samme effekt som Rambus, uden brug af anderledes arkitektur. Udgangspunktet var at lave modulet med en 64bit bus på 200MHz, og eftersom overførsler også her foregår både ved den stigende og faldende klokfrekvens ville den effektive hastighed blive 400MHz. Det giver en teoretisk båndbredde på 3.2GB per sekund, altså dobbelt så hurtigt som RDRAM. SLDRAM er også en åben standard og har hurtigere Burst Timings end RDRAM. På mange måde var potentiellet bedre end RDRAM, men den fik aldrig den brede accept hos chipsæt producenterne og blev trods dets potentielle, der på mange måder var større en RDRAM, en fiasko. Double Data Rate Synchronous DRAM (DDR SDRAM): DDR SDRAM bygger på den samme teknologi og arkitektur som det regulerer SDRAM. Forskellen er den dobbelte overførselshastighed, hvor DDR SDRAM, ligesom ved RDRAM, kan overføre både ved stigende og faldende klokfrekvens, og derved overføre to bits data til systembusen fra rammens I/O buffer per klokfrekvens, og derved den dobbelte overførsel i forhold til ordinær SDRAM, der kun kan overføre en bit per klokfrekvens. På den måde vil et SDRAM modul på 100MHz nu kunne yde 200Mhz. DDR RAM er i dag den mest udbredte RAM type, og er klart den der understøttes af flest bundkort producenter. Den fås i flere forskellige varianter, som kan ses i listen nedenfor: PC2100 (266 MHz) PC2700 (333 MHz) PC3200 (400 MHz) PC3500 (433 MHz) PC3700 (466 MHz) PC4000 (500 MHz) PC4200 (533 MHz) PC4300 (533 MHz) PC4400 (550 MHz) De forskellige varianter er faktisk bare et udtryk for den samme RAM type med forskellige hastigheder. Hvilken hastighed (variant) man skal vælge afhænger af hvad bundkortet understøtter. Ofte er det ikke noget problem i at købe en variant der har en højere hastighed en hvad ens bundkort understøtter, dog må man så acceptere ikke at udnytte rammens maksimale hastighed til fulde. Se vores teoretiske hastighedssammenligning længere nede i guiden. DDR SDRAM passer i en DIMM port, og har 184 pins. Double Data Rate Synchronous DRAM II (DDR SDRAM II): Som navnet antyder er DDR-II er en udvidelse af det traditionelle DDR RAM modul. DDR-II tager nogle af fordelene fra RAMBUS teknologien og kombinerer med DDR modulet, for derved at opnå en bedre ydeevne. Mens DDR-I RAM kan overføre 2 bits data på en klokfrekvens, kan DDR-II overføre det samme men på ½ Klokfrekvens, altså dobbelt så hurtigt. Et DDR modul med en 100 MHz klokfrekvens, vil altså effektivt afvikles på 200 MHz, mens det for et DDR-II modul vil være 400 MHz. At DDR-II RAM så i praktisk ikke er dobbelt så hurtig som dens forgænger har med ventetiden imellem selve hukommelsen og hukommelsecontrolleren at gøre. Nedenstående viser en sammenhæng mellem hastighederne for DDR og DDR-II: DDR: 100 MHz klokfrekvens -> 100 MHz Data buffer -> 200 MHz hastighed DDR-II: 100 MHz klokfrekvens -> 200 MHz Data buffer -> 400 MHz hastighed Enhanced SDRAM (ESDRAM): For at forøge effektiviteten og hastigheden af standard RAM moduler, har nogen producenter forsøgt sig med at inkorporere SRAM direkte på RAM modulet, for derved at lave et mindre cache lager. ESDRAM er altså principielt SDRAM med en lille smule cache hukommelse, til at gemme det mest benyttede data i. Cachen understøtter overførsler på op til 200Mhz, altså omkring dobbelt så hurtigt som resten af hukommelse, og derfor en hastighedsforøgelse for den samlede RAM klods. Passer i en DIMM port, og har 168 pins. Hukommelsesfrekvens / Tilgangstid: Denne såkaldte tilgangstid til hukommelsen måles i dag i MHz. Tidligere, før udviklingen af SDRAM blev det imidlertid målt i Nanosekunder. Denne overgang fra nanosekunder til Megahertz kunne godt give anledning til forvirring, men er efterhånden ikke et problem længere. Dog skal man lige være opmærksom på at hvor det førhen var bedst med en så lavt så mulig tilgangstid (målt i nanosekunder), er det i dag hvor måleenheden er frekvens bedst med et så højt tal som muligt. Bemærk at bundkortet skal understøtte de enkelte hastigheder. Eksempelvis kan man ikke bruge PC100 SDRAM i et bundkort der kræver PC133 SDRAM. Derimod kan man som regel uden problemer bruge hurtigere RAM i langsommere bundkort. Det anbefales altid at bruge den hurtigste RAM, som dit bundkort understøtter. Hver også opmærksom på at hvis man kombinere RAM med forskellig hastighed, vil alle RAM klodserne køre med den hastighed, som den langsomste klods har. Se længere nede i guiden vore teoretiske hastighedssammenligning. Burst timings (CAS): Burst timings er et udtryk de små pauser (Latency) der er indlagt i processen når CPU’en forespørger rammen for data til rammen sender resultatet til CPU’en. Ventetiderne er nødvendige for at elektronikken kan følge med, og at der ikke sker datafejl i overførslen. Disse ventetider har en relativ stor betydning for RAM modulets samlet ydeevne, og er derfor vigtige at tage i betragtning. Der er fire forskellige indlagte pauser, der bruges på forskellige tidspunkter i overførselsprocessen. Den vigtigste og mest betydelige er den såkaldte CAS Latency (CL). CAS er betegnelsen for den forsinkelse der er fra rammen modtager en instruks til den udfører den. Jo lavere denne er jo hurtigere opererer rammen. Den præcise sammenhæng mellem de forskellige Burst timings, og hvad der er den optimale indstilling, er en større videnskab, som ikke er medtaget i denne guide. Dette er nemlig også afhængigt af den præcise RAM model og det bundkort de skal monteres i. Som hovedregel kan det anbefales at købe RAM med en CAS Latency (CL) på 2.5, eller på 2.0 hvis man ønsker at eksperimentere med overclocking af computeren. Teoretisk Hastigheds Sammenligning: Når vi sammenligner RAM er der foruden de foromtalte Burst Timings tre begreber at holde styr på: Hastigheden (MHz) Data Bus Bredden Samlede båndbredde Hastighedsbegrebet er blevet beskrevet før i guiden. Data bus bredden fortæller hvor meget data der kan flyttes ad gangen, hvor eksempelvis 2x16 bit betyder at der er to data busser, som hver kan flytte 16 bit ad gangen. Til sidst er der så båndbredde, der konkret fortæller hvor mange Gigabyte rammen kan flytte per sekund. Båndbreddens størrelse afhænger direkte af hastigheden på RAM samt størrelsen på databussen. I denne tabel kan du sammenligne de forskellige typer RAM og deres hastigheder. Betegnelse RAM type Clock Data bus Båndbredde PC66 SDRAM 66MHz 64 bit 0,5GB/s PC100 SDRAM 100MHz 64 bit 0,8GB/s PC133 SDRAM 133MHz 64 bit 1,06GB/s PC1600 DDR200 100MHz 64 bit 1,6GB/s PC1600 DDR200 Dual 100MHz 2 x 64 bit 3,2GB/s PC2100 DDR266 133MHz 64 bit 2,1GB/s PC2100 DDR266 Dual 133MHz 2 x 64 bit 4,2GB/s PC2700 DDR333 166MHz 64 bit 2,7GB/s PC2700 DDR333 Dual 166MHz 2 x 64 bit 5,4GB/s PC3200 DDR400 200MHz 64 bit 3,2GB/s PC3200 DDR400 Dual 200MHz 2 x 64 bit 6,4GB/s PC4200 DDR533 266MHz 64 bit 4,2GB/s PC4200 DDR533 Dual 266MHz 2 x 64 bit 8,4GB/s PC800 RDRAM Dual 400MHz 2 x 16 bit 3,2GB/s PC1066 RDRAM Dual 533MHz 2 x 16 bit 4,2GB/s PC1200 RDRAM Dual 600MHz 2 x 16 bit 4,8GB/s PC800 RDRAM Dual 400MHz 2 x 32 bit 6,4GB/s PC1066 RDRAM Dual 533MHz 2 x 32 bit 8,4GB/s PC1200 RDRAM Dual 600MHz 2 x 32 bit 9,6GB/s De steder hvor DDR-RAM er angivet som dual gælder for de bundkort og chipsæt der understøtter Dual Channel RAM. Her kræves at man har to ens DDR-RAM klodser i, for at få udnytte af den dobbelte data-bus, og dermed båndbredde. Man kan altså regne med at DDR-RAM i de fleste bundkort ikke er Dual, og dette skal man være opmærksom på, når man slår op i tabellen. Hvad angår RDRAM, så er 16-bit udgaverne (som også kaldes PC1066 RDRAM) af natur Dual, da de skal monteres parvist. Der er også en anden type RDRAM på markedet, nemlig 32-bit udgaven. Disse kører 2x16 bit på en enkelt klods, og man behøver ikke installere dem parvist. De betegnes oftest som 32-bit RIMM4200 RAM. Dataintegritetskontrol (ECC RAM): Alle typer RAM kan i dag fås som ECC (Error Correction) RAM. Denne RAM type er dyrere, men til gengæld har de, ved hjælp af en ekstra indbygget chip, muligheden for at rette fejl under overførslen til og fra hukommelse. Almindelige RAM har ikke denne funktion. Det er sjældent at disse fejl opstår, og for almindelige brugere har det næppe den store betydning. ECC RAM anbefales derimod til computere, hvor det er vigtigt med høj sikkerhed og stabilitet – for eksempel til servere. Desuden er ECC RAM også lidt langsommere end almindelige RAM, pga. at der skal sendes fejlrettelsesdata, sammen med almindelige data. Prosessor http://student.iu.hio.no/~s127645/webprosjekt/content/prosessor.php En 64-bit prosessor fra AMD En mikroprosessor er en brikke som innholder veldig mange transistorer. Brikken er bygd opp lagvis med silikon og halvleder metallet silisium, Transistorene er laget av silisium. Den første mikroprosessoren som ble laget, var Intel sin 4004 som ble introdusert i 1971. 4004 brikken var ikke spesielt kraftig, alt den kunne gjøre var å addere og subtrahere og kunne bare gjør det 4 bits om gangen. Det banebrytende med 4004 brikken var at det var en databrikke. Før 4004 brikken kom ut ble hjerte av datamaskinen bygd ved å lodde sammen en og en transistor om gangen. Dette gjorde at størrelsen ble drastisk redusert og banet vei for den første lommekalkulatoren. Den første mikroprosessoren som fant veien inn i en PC var 8080 brikken. Prosessoren ble laget av Intel og introdusert i 1974. Men det var ikke før 1982 at Intel sin prosessor ble bygd inn i en IBM PC og start hele PC revolusjonen. Alle prosessorene som har blitt laget av Intel er basert på 8088 brikken. Noen forbedringer, men stort sett det samme. Dagens Pentium 4 kan kjøre alle typene av kode som ble kjørt på 8088 brikken, men den gjør det 5000 ganger raskere. Hvordan virker en mikroprosessor? Her er hovedaktivitetene som skjer i en mikroprosessor: Ved å bruke ALU (Arithmetic/Logic Unit) kan en mikroprosessor kan utføre mattematiske operasjoner som addisjon, subtraksjon, multiplikasjon og divisjon. Moderne mikroprosessorer innholder komplette avanserte prosessorer som kan utføre ekstremt kompliserte operasjoner på store tall. En mikroprosessor kan flytte data fra en lokasjon i minne til et annet. En mikroprosessor kan ta avgjørelser og hoppe til en ny pakke med instruksjoner basert på de avgjørelsene. En mikroprosessor innholder busser som sender og mottar data fra minne. Disse bussene heter adresse buss og data buss. Adresse bussen sender adresser til minne der data skal plasseres. Data bussen sender og mottar data fra minne. Signalene blir sendt parallelt og bussene er, i dagens mikroprosessorer, 32 bit. Litt om fremtiden; 64 bit mikroprosessor har vært her siden 1992, men det er først i de siste årene at de har kommet ut på markedet. Og 64 bit prosessorene kommer til å dominere markedet framover. Dagens mikroprosessorer innholder et intern minne som er på mellom 512Kb og 2Mb. Det vil i løpet av kort tid komme mikroprosessorer med 3, 4, 6, 9 og 18Mb internminne. Dagens raskeste mikroprosessorer har en klokkehastighet på 3,6GHz, om kort vil det komme ut mikroprosessorer med opp til 10GHz. HARDDISC http://www.hitachigst.com/hdd/research/ Hvordan virker en CD-ROM? http://www.netprofessor.dk/artikler.asp?id=95 Nedenstående er en meget simpel gennemgang af en meget kompleks funktion. Nogle kan stadig huske da CD’er stadig blev kaldt Laser Disc. Grunden til man kaldte dem for Laser Disc, er fordi man benytter laser teknologi til at læse informationerne på CD’erne. En CD er primært fremstillet i reflekterende aluminium med et beskyttende lag plastic omkring. I aluminiumet er der ætset eller raderet en spiralspor der er ca. 4,83 km. langt. Spiralsporet indeholder det information, som ligger på CD’en, det kan være musik, computerspil m.m. Spiralsporet har to forskellige slags overflader, en flad og et hul. Disse to overflader indeholder information om de 0 og 1-taller, der udgør den binære kode. Laserstrålen bliver rettet mod CD’en og kan enten ramme en flad eller et hul, når strålen rammer en flad, bliver strålen brudt og når den rammer et hul, bliver den reflekteret til en lysfølsom sensor. Pulsen, som rammer sensoren, generer en ganske lille elektrisk strøm. Således bliver signalet til en serie puls og ingen puls; 0’erne og 1’erne. Omdrejningerne af CD’en og laserens position er justeret, således informationen kan blive læst jævnt. Disketter http://www.webopedia.com/TERM/F/floppy_disk.html A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5¼-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable. Floppies come in three basic sizes: 8-inch: The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop computer does not use the 8-inch floppy disk. 5¼-inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB. 3½-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB. CISC og RISC prosessorer http://www.amigau.com/aig/riscisc.html RISC The concept was developed by John Cocke of IBM Research during 1974. His argument was based upon the notion that a computer uses only 20% of the instructions, making the other 80% superfluous to requirement. A processor based upon this concept would use few instructions, which would require fewer transistors, and make them cheaper to manufacture. By reducing the number of transistors and instructions to only those most frequently used, the computer would get more done in a shorter amount of time. The term 'RISC' (short for Reduced Instruction Set Computer) was later coined by David Patterson, a teacher at the University of California in Berkeley. The RISC concept was used to simplify the design of the IBM PC/XT, and was later used in the IBM RISC System/6000 and Sun Microsystems' SPARC microprocessors. The latter CPU led to the founding of MIPS Technologies, who developed the M.I.P.S. RISC microprocessor (Microprocessor without Interlocked Pipe Stages). Many of the MIPS architects also played an instrumental role in the creation of the Motorola 68000, as used in the first Amigas (MIPS Technologies were later bought by Silicon Graphics).. The MIPS processor has continued development, remaining a popular choice in embedded and low-end market. At one time, it was suspected the Amiga MCC would use this CPU to reduce the cost of manufacture. However, the consumer desktop market is limited, only the PowerPC processor remains popular in the choice of RISC alternatives. This is mainly due to Apple's continuous use of the series for its PowerMac range. CISC CISC (Complex Instruction Set Computer) is a retroactive definition that was introduced to distinguish the design from RISC microprocessors. In contrast to RISC, CISC chips have a large amount of different and complex instruction. The argument for its continued use indicates that the chip designers should make life easier for the programmer by reducing the amount of instructions required to program the CPU. Due to the high cost of memory and storage CISC microprocessors were considered superior due to the requirements for small, fast code. In an age of dwindling memory hard disk prices, code size has become a non-issue (MS Windows, hello?). However, CISC-based systems still cover the vast majority of the consumer desktop market. The majority of these systems are based upon the x86 architecture or a variant. The Amiga, Atari, and pre-1994 Macintosh systems also use a CISC microprocessor. RISC Vs. CISC The argument over which concept is better has been repeated over the past few years. Macintosh owners have elevated the argument to a pseudo religious level in support of their RISC-based God (the PowerPC sits next to the Steve Jobs statue on every Mac altar). Both positions have been blurred by the argument that we have entered a Post-RISC stage. RISC: For and Against RISC supporters argue that it the way of the future, producing faster and cheaper processors - an Apple Mac G3 offers a significant performance advantage over its Intel equivalent. Instructions are executed over 4x faster providing a significant performance boost! However, RISC chips require more lines of code to produce the same results and are increasingly complex. This will increase the size of the application and the amount of overhead required. RISC developers have also failed to remain in competition with CISC alternatives. The Macintosh market has been damaged by several problems that have affected the availability of 500MHz+ PowerPC chips. In contrast, the PC compatible market has stormed ahead and has broken the 1GHz barrier. Despite the speed advantages of the RISC processor, it cannot compete with a CISC CPU that boasts twice the number of clock cycles. CISC: For and Against As discussed above, CISC microprocessors are more expensive to make than their RISC cousins. However, the average Macintosh is more expensive than the WIntel PC. This is caused by one factor that the RISC manufacturers have no influence over - market factors. In particular, the WIntel market has become the definition of personal computing, creating a demand from people who have not used a computer previous. The x86 market has been opened by the development of several competing processors, from the likes of AMD, Cyrix, and Intel. This has continually reduced the price of a CPU of many months. In contrast, the PowerPC Macintosh market is dictated by Apple. This reduces the cost of x86 - based microprocessors, while the PowerPC market remains stagnant. Post-RISC As the world enters the 21st century the CISC Vs. RISC arguments have been swept aside by the recognition that neither terms are accurate in their description. The definition of 'Reduced' and 'Complex' instructions has begun to blur, RISC chips have increased in their complexity (compare the PPC 601 to the G4 as an example) and CISC chips have become more efficient. The result are processors that are defined as RISC or CISC only by their ancestry. The PowerPC 601, for example, supports more instructions than the Pentium. Yet the Pentium is a CISC chip, while the 601 is considered to be RISC. CISC chips have also gained techniques associated with RISC processors. Intel describe the Pentium II as a CRISC processor, while AMD use a RISC architecture but remain compatible with the dominant x86 CISC processors. Thus it is no longer important which camp the processor comes from, the emphasis has once-again been placed upon the operating system and the speed that it can execute instructions. EPIC In the aftermath of the CISC-RISC conflict, a new enemy has appeared to threaten the peace. EPIC (Explicitly Parallel Instruction Computing) was developed by Intel for the server market, thought it will undoubtedly appear in desktops over the next few years. The first EPIC processor will be the 64-bit Merced, due for release sometime during 2001 (or 2002, 2003, etc.). The market may be divided between combined CISC-RISC systems in the low-end and EPIC in the high-end. Famous RISC microprocessors 801 To prove that his RISC concept was sound, John Cocke created the 801 prototype microprocessor (1975). It was never marketed but plays a pivotal role in computer history, becoming the first RISC microprocessor. RISC 1 and 2 The first "proper" RISC chips were created at Berkeley University in 1985. ARM One of the most well known RISC developers is Cambridge based Advanced Research Machines (originally Acorn Research Machines). Their ARM and StrongARM chips power the old Acorn Archimedes and the Apple Newton handwriting recognition systems. Since the unbundling of ARM from Acorn, Intel have invested a considerable amount of money in the company and have utilized the technology in their processor design. One of the main advantages for the ARM is the price- it costs less than £10. If Samsung had bought the Amiga in 1994, they would possibly have used the chip to power the low-end Amigas. SCANNER http://computer.howstuffworks.com/scanner.htm Scanners have become an important part of the home office over the last few years. Scanner technology is everywhere and used in many ways: Flatbed scanners, also called desktop scanners, are the most versatile and commonly used scanners. In fact, this article will focus on the technology as it relates to flatbed scanners. Sheet-fed scanners are similar to flatbed scanners except the document is moved and the scan head is immobile. A sheet-fed scanner looks a lot like a small portable printer. Handheld scanners use the same basic technology as a flatbed scanner, but rely on the user to move them instead of a motorized belt. This type of scanner typically does not provide good image quality. However, it can be useful for quickly capturing text. Drum scanners are used by the publishing industry to capture incredibly detailed images. They use a technology called a photomultiplier tube (PMT). In PMT, the document to be scanned is mounted on a glass cylinder. At the center of the cylinder is a sensor that splits light bounced from the document into three beams. Each beam is sent through a color filter into a photomultiplier tube where the light is changed into an electrical signal. The basic principle of a scanner is to analyze an image and process it in some way. Image and text capture (optical character recognition or OCR) allow you to save information to a file on your computer. You can then alter or enhance the image, print it out or use it on your Web page. In this article, we'll be focusing on flatbed scanners, but the basic principles apply to most other scanner technologies. You will learn about the different types of scanners, how the scanning mechanism works and what TWAIN means. You will also learn about resolution, interpolation and bit depth. On the next page, you will learn about the various parts of a flatbed scanner. Anatomy of a Scanner Parts of a typical flatbed scanner include: Charge-coupled device (CCD) array Mirrors Scan head Glass plate Lamp Lens Cover Filters Stepper motor Stabilizer bar Belt Power supply Interface port(s) Control circuitry The core component of the scanner is the CCD array. CCD is the most common technology for image capture in scanners. CCD is a collection of tiny light-sensitive diodes, which convert photons (light) into electrons (electrical charge). These diodes are called photosites. In a nutshell, each photosite is sensitive to light -- the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site. Photons hitting a photosite and creating electrons The image of the document that you scan reaches the CCD array through a series of mirrors, filters and lenses. The exact configuration of these components will depend on the model of scanner, but the basics are pretty much the same. On the next page, you will see just how all the pieces of the scanner work together. Here are the steps that a scanner goes through when it scans a document: The document is placed on the glass plate and the cover is closed. The inside of the cover in most scanners is flat white, although a few are black. The cover provides a uniform background that the scanner software can use as a reference point for determining the size of the document being scanned. Most flatbed scanners allow the cover to be removed for scanning a bulky object, such as a page in a thick book. A lamp is used to illuminate the document. The lamp in newer scanners is either a cold cathode fluorescent lamp (CCFL) or a xenon lamp, while older scanners may have a standard fluorescent lamp. The entire mechanism (mirrors, lens, filter and CCD array) make up the scan head. The scan head is moved slowly across the document by a belt that is attached to a stepper motor. The scan head is attached to a stabilizer bar to ensure that there is no wobble or deviation in the pass. Pass means that the scan head has completed a single complete scan of the document. The image of the document is reflected by an angled mirror to another mirror. In some scanners, there are only two mirrors while others use a three mirror approach. Each mirror is slightly curved to focus the image it reflects onto a smaller surface. The last mirror reflects the image onto a lens. The lens focuses the image through a filter on the CCD array. The filter and lens arrangement vary based on the scanner. Some scanners use a three pass scanning method. Each pass uses a different color filter (red, green or blue) between the lens and CCD array. After the three passes are completed, the scanner software assembles the three filtered images into a single full-color image. Most scanners today use the single pass method. The lens splits the image into three smaller versions of the original. Each smaller version passes through a color filter (either red, green or blue) onto a discrete section of the CCD array. The scanner combines the data from the three parts of the CCD array into a single full-color image. Another imaging array technology that has become popular in inexpensive flatbed scanners is contact image sensor (CIS). CIS replaces the CCD array, mirrors, filters, lamp and lens with rows of red, green and blue light emitting diodes (LEDs). The image sensor mechanism, consisting of 300 to 600 sensors spanning the width of the scan area, is placed very close to the glass plate that the document rests upon. When the image is scanned, the LEDs combine to provide white light. The illuminated image is then captured by the row of sensors. CIS scanners are cheaper, lighter and thinner, but do not provide the same level of quality and resolution found in most CCD scanners. We will take a look at what happens between the computer and scanner, but first let's talk about resolution. Resolution and Interpolation Scanners vary in resolution and sharpness. Most flatbed scanners have a true hardware resolution of at least 300x300 dots per inch (dpi). The scanner's dpi is determined by the number of sensors in a single row (x-direction sampling rate) of the CCD or CIS array by the precision of the stepper motor (y-direction sampling rate). For example, if the resolution is 300x300 dpi and the scanner is capable of scanning a letter-sized document, then the CCD has 2,550 sensors arranged in each horizontal row. A single-pass scanner would have three of these rows for a total of 7,650 sensors. The stepper motor in our example is able to move in increments equal to 1/300ths of an inch. Likewise, a scanner with a resolution of 600x300 has a CCD array with 5,100 sensors in each horizontal row. Sharpness depends mainly on the quality of the optics used to make the lens and the brightness of the light source. A bright xenon lamp and high-quality lens will create a much clearer, and therefore sharper, image than a standard fluorescent lamp and basic lens. Of course, many scanners proclaim resolutions of 4,800x4,800 or even 9,600x9,600. To achieve a hardware resolution with a x-direction sampling rate of 9,600 would require a CCD array of 81,600 sensors. If you look at the specifications, these high resolutions are usually labeled software- enhanced, interpolated resolution or something similar. What does that mean? Interpolation is a process that the scanning software uses to increase the perceived resolution of an image. It does this by creating extra pixels in between the ones actually scanned by the CCD array. These extra pixels are an average of the adjacent pixels. For example, if the hardware resolution is 300x300 and the interpolated resolution is 600x300, then the software is adding a pixel between every one scanned by a CCD sensor in each row. Another term used when talking about scanners is bit depth, also called color depth. This simply refers to the number of colors that the scanner is capable of reproducing. Each pixel requires 24 bits to create standard true color and virtually all scanners on the market support this. Many of them offer bit depths of 30 or 36 bits. They still only output in 24-bit color, but perform internal processing to select the best possible choice out of the colors available in the increased palette. There are many opinions about whether there is a noticeable difference in quality between 24-, 30- and 36-bit scanners. Image Transfer Scanning the document is only one part of the process. For the scanned image to be useful, it must be transferred to your computer. There are three common connections used by scanners: Parallel - Connecting through the parallel port is the slowest transfer method available. Small Computer System Interface (SCSI) - SCSI requires a special SCSI connection. Most SCSI scanners include a dedicated SCSI card to insert into your computer and connect the scanner to, but you can use a standard SCSI controller instead. Universal Serial Bus (USB) - USB scanners combine good speed, ease of use and affordability in a single package. FireWire - Usually found on higher-end scanners,FireWire connections are faster than USB and SCSI. FireWire is ideal for scanning high-resolution images. Did You Know? TWAIN is not an acronym. It actually comes from the phrase "Never the twain shall meet" because the driver is the go-between for the software and the scanner. Because computer people feel a need to make an acronym out of every term, TWAIN is known as Technology Without An Interesting Name! On your computer, you need software, called a driver, that knows how to communicate with the scanner. Most scanners speak a common language, TWAIN. The TWAIN driver acts as an interpreter between any application that supports the TWAIN standard and the scanner. This means that the application does not need to know the specific details of the scanner in order to access it directly. For example, you can choose to acquire an image from the scanner from within Adobe Photoshop because Photoshop supports the TWAIN standard. In addition to the driver, most scanners come with other software. Typically, a scanning utility and some type of image editing application are included. A lot of scanners include OCR software. OCR allows you to scan in words from a document and convert them into computer-based text. It uses an averaging process to determine what the shape of a character is and match it to the correct letter or number. The great thing about scanner technology today is that you can get exactly what you need. You can find a decent scanner with good software for less than $200, or get a fantastic scanner with incredible software for less than $1,000. It all depends on your needs and budget. For more information on scanners and related topics, check out the links on the next page. LASERSKRIVER http://computer.howstuffworks.com/laser-printer.htm The term inkjet printer is very descriptive of the process at work -- these printers put an image on paper using tiny jets of ink. The term laser printer, on the other hand, is a bit more mysterious -- how can a laser beam, a highly focused beam of light, write letters and draw pictures on paper? In this article, we'll unravel the mystery behind the laser printer, tracing a page's path from the characters on your computer screen to printed letters on paper. As it turns out, the laser printing process is based on some very basic scientific principles applied in an exceptionally innovative way. The Basics: Static Electricity The primary principle at work in a laser printer is static electricity, the same energy that makes clothes in the dryer stick together or a lightning bolt travel from a thundercloud to the ground. Static electricity is simply an electrical charge built up on an insulated object, such as a balloon or your body. Since oppositely charged atoms are attracted to each other, objects with opposite static electricity fields cling together. A laser printer uses this phenomenon as a sort of "temporary glue." The core component of this system is the photoreceptor, typically a revolving drum or cylinder. This drum assembly is made out of highly photoconductive material that is discharged by light photons. The Basics: Drum Initially, the drum is given a total positive charge by the charge corona wire, a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.) As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser "draws" the letters and images to be printed as a pattern of electrical charges -- an electrostatic image. The system can also work with the charges reversed -- that is, a positive electrostatic image on a negative background. After the pattern is set, the printer coats the drum with positively charged toner -- a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum, but not to the positively charged "background." This is something like writing on a soda can with glue and then rolling it over some flour: The flour only sticks to the glue-coated part of the can, so you end up with a message written in powder. With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire (charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, it is discharged by the detac corona wire immediately after picking up the toner. The Basics: Fuser Finally, the printer passes the paper through the fuser, a pair of heated rollers. As the paper passes through these rollers, the loose toner powder melts, fusing with the fibers in the paper. The fuser rolls the paper to the output tray, and you have your finished page. The fuser also heats up the paper itself, of course, which is why pages are always hot when they come out of a laser printer or photocopier. So what keeps the paper from burning up? Mainly, speed -- the paper passes through the rollers so quickly that it doesn't get very hot. After depositing toner on the paper, the drum surface passes the discharge lamp. This bright light exposes the entire photoreceptor surface, erasing the electrical image. The drum surface then passes the charge corona wire, which reapplies the positive charge. Conceptually, this is all there is to it. Of course, actually bringing everything together is a lot more complex. In the following sections, we'll examine the different components in greater detail to see how they produce text and images so quickly and precisely. The Controller: The Conversation Before a laser printer can do anything else, it needs to receive the page data and figure out how it's going to put everything on the paper. This is the job of the printer controller. The printer controller is the laser printer's main onboard computer. It talks to the host computer (for example, your PC) through a communications port, such as a parallel port or USB port. At the start of the printing job, the laser printer establishes with the host computer how they will exchange data. The controller may have to start and stop the host computer periodically to process the information it has received. A typical laser printer has a few different types of communications ports. In an office, a laser printer will probably be connected to several separate host computers, so multiple users can print documents from their machine. The controller handles each one separately, but may be carrying on many "conversations" concurrently. This ability to handle several jobs at once is one of the reasons why laser printers are so popular. The Controller: The Language For the printer controller and the host computer to communicate, they need to speak the same page description language. In earlier printers, the computer sent a special sort of text file and a simple code giving the printer some basic formatting information. Since these early printers had only a few fonts, this was a very straightforward process. These days, you might have hundreds of different fonts to choose from, and you wouldn't think twice about printing a complex graphic. To handle all of this diverse information, the printer needs to speak a more advanced language. The primary printer languages these days are Hewlett Packard's Printer Command Language (PCL) and Adobe's Postscript. Both of these languages describe the page in vector form -- that is, as mathematical values of geometric shapes, rather than as a series of dots (a bitmap image). The printer itself takes the vector images and converts them into a bitmap page. With this system, the printer can receive elaborate, complex pages, featuring any sort of font or image. Also, since the printer creates the bitmap image itself, it can use its maximum printer resolution. Some printers use a graphical device interface (GDI) format instead of a standard PCL. In this system, the host computer creates the dot array itself, so the controller doesn't have to process anything -- it just sends the dot instructions on to the laser. But in most laser printers, the controller must organize all of the data it receives from the host computer. This includes all of the commands that tell the printer what to do -- what paper to use, how to format the page, how to handle the font, etc. For the controller to work with this data, it has to get it in the right order. The Controller: Setting up the Page Once the data is structured, the controller begins putting the page together. It sets the text margins, arranges the words and places any graphics. When the page is arranged, the raster image processor (RIP) takes the page data, either as a whole or piece by piece, and breaks it down into an array of tiny dots. As we'll see in the next section, the printer needs the page in this form so the laser can write it out on the photoreceptor drum. In most laser printers, the controller saves all print-job data in its own memory. This lets the controller put different printing jobs into a queue so it can work through them one at a time. It also saves time when printing multiple copies of a document, since the host computer only has to send the data once. The Laser Assembly Since it actually draws the page, the printer's laser system -- or laser scanning assembly -- must be incredibly precise. The traditional laser scanning assembly includes: A laser A movable mirror A lens The laser receives the page data -- the tiny dots that make up the text and images -- one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every dot to be printed, and no pulse for every dot of empty space. The laser doesn't actually move the beam itself. It bounces the beam off a movable mirror instead. As the mirror moves, it shines the beam through a series of lenses. This system compensates for the image distortion caused by the varying distance between the mirror and points along the drum. Writing the Page The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer moves the photoreceptor drum up a notch so the laser assembly can draw the next line. A small print-engine computer synchronizes all of this perfectly, even at dizzying speeds. Some laser printers use a strip of light emitting diodes (LEDs) to write the page image, instead of a single laser. Each dot position has its own dedicated light, which means the printer has one set print resolution. These systems cost less to manufacture than true laser assemblies, but they produce inferior results. Typically, you'll only find them in less expensive printers. Photocopiers Laser printers work the same basic way as photocopiers, with a few significant differences. The most obvious difference is the source of the image: A photocopier scans an image by reflecting a bright light off of it, while a laser printer receives the image in digital form. Another major difference is how the electrostatic image is created. When a photocopier bounces light off a piece of paper, the light reflects back onto the photoreceptor from the white areas but is absorbed by the dark areas. In this process, the "background" is discharged, while the electrostatic image retains a positive charge. This method is called "write-white." In most laser printers, the process is reversed: The laser discharges the lines of the electrostatic image and leaves the background positively charged. In a printer, this "write-black" system is easier to implement than a "write-white" system, and it generally produces better results. Toner Basics One of the most distinctive things about a laser printer (or photocopier) is the toner. It's such a strange concept for the paper to grab the "ink" rather than the printer applying it. And it's even stranger that the "ink" isn't really ink at all. So what is toner? The short answer is: It's an electrically-charged powder with two main ingredients: pigment and plastic. The role of the pigment is fairly obvious -- it provides the coloring (black, in a monochrome printer) that fills in the text and images. This pigment is blended into plastic particles, so the toner will melt when it passes through the heat of the fuser. This quality gives toner a number of advantages over liquid ink. Chiefly, it firmly binds to the fibers in almost any type of paper, which means the text won't smudge or bleed easily. Applying Toner So how does the printer apply this toner to the electrostatic image on the drum? The powder is stored in the toner hopper, a small container built into a removable casing. The printer gathers the toner from the hopper with the developer unit. The "developer" is actually a collection of small, negatively charged magnetic beads. These beads are attached to a rotating metal roller, which moves them through the toner in the toner hopper. Because they are negatively charged, the developer beads collect the positive toner particles as they pass through. The roller then brushes the beads past the drum assembly. The electrostatic image has a stronger negative charge than the developer beads, so the drum pulls the toner particles away In a lot of printers, the toner hopper, developer and drum assembly are combined in one replaceable cartridge. The drum then moves over the paper, which has an even stronger charge and so grabs the toner. After collecting the toner, the paper is immediately discharged by the detac corona wire. At this point, the only thing keeping the toner on the page is gravity -- if you were to blow on the page, you would completely lose the image. The page must pass through the fuser to affix the toner. The fuser rollers are heated by internal quartz tube lamps, so the plastic in the toner melts as it passes through. But what keeps the toner from collecting on the fuser rolls, rather than sticking to the page? To keep this from happening, the fuser rolls must be coated with Teflon, the same non-stick material that keeps your breakfast from sticking to the bottom of the frying pan. Color Printers Initially, most commercial laser printers were limited to monochrome printing (black writing on white paper). But now, there are lots of color laser printers on the market. Essentially, color printers work the same way as monochrome printers, except they go through the entire printing process four times -- one pass each for cyan (blue), magenta (red), yellow and black. By combining these four colors of toner in varying proportions, you can generate the full spectrum of color. There are several different ways of doing this. Some models have four toner and developer units on a rotating wheel. The printer lays down the electrostatic image for one color and puts that toner unit into position. It then applies this color to the paper and goes through the process again for the next color. Some printers add all four colors to a plate before placing the image on paper. Some more expensive printers actually have a complete printer unit -- a laser assembly, a drum and a toner system -- for each color. The paper simply moves past the different drum heads, collecting all the colors in a sort of assembly line. Advantages of a Laser So why get a laser printer rather than a cheaper inkjet printer? The main advantages of laser printers are speed, precision and economy. A laser can move very quickly, so it can "write" with much greater speed than an ink jet. And because the laser beam has an unvarying diameter, it can draw more precisely, without spilling any excess ink. Laser printers tend to be more expensive than inkjet printers, but it doesn't cost as much to keep them running -- toner powder is cheap and lasts a long time, while you can use up expensive ink cartridges very quickly. This is why offices typically use a laser printer as their "work horse," their machine for printing long text documents. In most models, this mechanical efficiency is complemented by advanced processing efficiency. A typical laser-printer controller can serve everybody in a small office. When they were first introduced, laser printers were too expensive to use as a personal printer. Since that time, however, laser printers have gotten much more affordable. Now you can pick up a basic model for just a little bit more than a nice inkjet printer. As technology advances, laser-printer prices should continue to drop, while performance improves. We'll also see a number of innovative design variations, and possibly brand-new applications of electrostatic printing. Many inventors believe we've only scratched the surface of what we can do with simple static electricity! For more information on laser printers and related topics, check out the links on the next page. FARGESKRIVER http://computer.howstuffworks.com/inkjet-printer.htm No matter where you are reading this article from, you most likely have a printer nearby. And there's a very good chance that it is an inkjet printer. Since their introduction in the latter half of the 1980s, inkjet printers have grown in popularity and performance while dropping significantly in price. An inkjet printer is any printer that places extremely small droplets of ink onto paper to create an image. If you ever look at a piece of paper that has come out of an inkjet printer, you know that: The dots are extremely small (usually between 50 and 60 microns in diameter), so small that they are tinier than the diameter of a human hair (70 microns)! The dots are positioned very precisely, with resolutions of up to 1440x720 dots per inch (dpi). The dots can have different colors combined together to create photo-quality images. In this edition of HowStuffWorks, you will learn about the various parts of an inkjet printer and how these parts work together to create an image. You will also learn about the ink cartridges and the special paper some inkjet printers use. First, let's take a quick look at the various printer technologies. Impact vs. Non-impact There are several major printer technologies available. These technologies can be broken down into two main categories with several types in each: Impact - These printers have a mechanism that touches the paper in order to create an image. There are two main impact technologies: Dot matrix printers use a series of small pins to strike a ribbon coated with ink, causing the ink to transfer to the paper at the point of impact. Character printers are basically computerized typewriters. They have a ball or series of bars with actual characters (letters and numbers) embossed on the surface. The appropriate character is struck against the ink ribbon, transferring the character's image to the paper. Character printers are fast and sharp for basic text, but very limited for other use. Non-impact - These printers do not touch the paper when creating an image. Inkjet printers are part of this group, which includes: Inkjet printers, which are described in this article, use a series of nozzles to spray drops of ink directly on the paper. Laser printers, covered in-depth in How Laser Printers Work, use dry ink (toner), static electricity, and heat to place and bond the ink onto the paper. Solid ink printers contain sticks of wax-like ink that are melted and applied to the paper. The ink then hardens in place. Dye-sublimation printers have a long roll of transparent film that resembles sheets of red-, blue-, yellow- and gray-colored cellophane stuck together end to end. Embedded in this film are solid dyes corresponding to the four basic colors used in printing: cyan, magenta, yellow and black (CMYK). The print head uses a heating element that varies in temperature, depending on the amount of a particular color that needs to be applied. The dyes vaporize and permeate the glossy surface of the paper before they return to solid form. The printer does a complete pass over the paper for each of the basic colors, gradually building the image. Thermal wax printers are something of a hybrid of dye-sublimation and solid ink technologies. They use a ribbon with alternating CMYK color bands. The ribbon passes in front of a print head that has a series of tiny heated pins. The pins cause the wax to melt and adhere to the paper, where it hardens in place. Thermal autochrome printers have the color in the paper instead of in the printer. There are three layers (cyan, magenta and yellow) in the paper, and each layer is activated by the application of a specific amount of heat. The print head has a heating element that can vary in temperature. The print head passes over the paper three times, providing the appropriate temperature for each color layer as needed. Out of all of these incredible technologies, inkjet printers are by far the most popular. In fact, the only technology that comes close today is laser printers. So, let's take a closer look at what's inside an inkjet printer. Inside an Inkjet Printer Parts of a typical inkjet printer include: Print head assembly Print head - The core of an inkjet printer, the print head contains a series of nozzles that are used to spray drops of ink. Ink cartridges - Depending on the manufacturer and model of the printer, ink cartridges come in various combinations, such as separate black and color cartridges, color and black in a single cartridge or even a cartridge for each ink color. The cartridges of some inkjet printers include the print head itself. Print head stepper motor - A stepper motor moves the print head assembly (print head and ink cartridges) back and forth across the paper. Some printers have another stepper motor to park the print head assembly when the printer is not in use. Parking means that the print head assembly is restricted from accidentally moving, like a parking brake on a car. Belt - A belt is used to attach the print head assembly to the stepper motor. Stabilizer bar - The print head assembly uses a stabilizer bar to ensure that movement is precise and controlled. Paper tray/feeder - Most inkjet printers have a tray that you load the paper into. Some printers dispense with the standard tray for a feeder instead. The feeder typically snaps open at an angle on the back of the printer, allowing you to place paper in it. Feeders generally do not hold as much paper as a traditional paper tray. Rollers - A set of rollers pull the paper in from the tray or feeder and advance the paper when the print head assembly is ready for another pass. Paper feed stepper motor - This stepper motor powers the rollers to move the paper in the exact increment needed to ensure a continuous image is printed. Power supply - While earlier printers often had an external transformer, most printers sold today use a standard power supply that is incorporated into the printer itself. Control circuitry - A small but sophisticated amount of circuitry is built into the printer to control all the mechanical aspects of operation, as well as decode the information sent to the printer from the computer. Interface port(s) - The parallel port is still used by many printers, but most newer printers use the USB port. A few printers connect using a serial port or small computer system interface (SCSI) port. Heat vs. Vibration Different types of inkjet printers form their droplets of ink in different ways. There are two main inkjet technologies currently used by printer manufacturers: Thermal bubble - Used by manufacturers such as Canon and Hewlett Packard, this method is commonly referred to as bubble jet. In a thermal inkjet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble "pops" (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously. Click the button to see how a thermal bubble inkjet printer works. Piezoelectric - Patented by Epson, this technology uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed out. Click on the button to see how a piezoelectric inkjet printer works. Let's walk through the printing process to see just what happens. Click "OK" to Print When you click on a button to print, there is a sequence of events that take place: The software application you are using sends the data to be printed to the printer driver. The driver translates the data into a format that the printer can understand and checks to see that the printer is online and available to print. The data is sent by the driver from the computer to the printer via the connection interface (parallel, USB, etc.). The printer receives the data from the computer. It stores a certain amount of data in a buffer. The buffer can range from 512 KB random access memory (RAM) to 16 MB RAM, depending on the model. Buffers are useful because they allow the computer to finish with the printing process quickly, instead of having to wait for the actual page to print. A large buffer can hold a complex document or several basic documents. If the printer has been idle for a period of time, it will normally go through a short clean cycle to make sure that the print head(s) are clean. Once the clean cycle is complete, the printer is ready to begin printing. The control circuitry activates the paper feed stepper motor. This engages the rollers, which feed a sheet of paper from the paper tray/feeder into the printer. A small trigger mechanism in the tray/feeder is depressed when there is paper in the tray or feeder. If the trigger is not depressed, the printer lights up the "Out of Paper" LED and sends an alert to the computer. Once the paper is fed into the printer and positioned at the start of the page, the print head stepper motor uses the belt to move the print head assembly across the page. The motor pauses for the merest fraction of a second each time that the print head sprays dots of ink on the page and then moves a tiny bit before stopping again. This stepping happens so fast that it seems like a continuous motion. Multiple dots are made at each stop. It sprays the CMYK colors in precise amounts to make any other color imaginable. At the end of each complete pass, the paper feed stepper motor advances the paper a fraction of an inch. Depending on the inkjet model, the print head is reset to the beginning side of the page, or, in most cases, simply reverses direction and begins to move back across the page as it prints. This process continues until the page is printed. The time it takes to print a page can vary widely from printer to printer. It will also vary based on the complexity of the page and size of any images on the page. For example, a printer may be able to print 16 pages per minute (PPM) of black text but take a couple of minutes to print one, full-color, page-sized image. Once the printing is complete, the print head is parked. The paper feed stepper motor spins the rollers to finish pushing the completed page into the output tray. Most printers today use inks that are very fast-drying, so that you can immediately pick up the sheet without smudging it. In the next section, you will learn a little more about the ink cartridges and the paper used. Paper and Ink Inkjet printers are fairly inexpensive. They cost less than a typical black-and-white laser printer, and much less than a color laser printer. In fact, quite a few of the manufacturers sell some of their printers at a loss. Quite often, you can find the printer on sale for less than you would pay for a set of the ink cartridges! Why would they do this? Because they count on the supplies you purchase to provide their profit. This is very similar to the way the video game business works. The hardware is sold at or below cost. Once you buy a particular brand of hardware, then you must buy the other products that work with that hardware. In other words, you can't buy a printer from Manufacturer A and ink cartridges from Manufacturer B. They will not work together. This cartridge has cyan, magenta and yellow inks in separate reservoirs. Another way that they have reduced costs is by incorporating much of the actual print head into the cartridge itself. The manufacturers believe that since the print head is the part of the printer that is most likely to wear out, replacing it every time you replace the cartridge increases the life of the printer. The paper you use on an inkjet printer greatly determines the quality of the image. Standard copier paper works, but doesn't provide as crisp and bright an image as paper made for an inkjet printer. There are two main factors that affect image quality: Brightness Absorption The brightness of a paper is normally determined by how rough the surface of the paper is. A course or rough paper will scatter light in several directions, whereas a smooth paper will reflect more of the light back in the same direction. This makes the paper appear brighter, which in turn makes any image on the paper appear brighter. You can see this yourself by comparing a photo in a newspaper with a photo in a magazine. The smooth paper of the magazine page reflects light back to your eye much better than the rough texture of the newspaper. Any paper that is listed as being bright is generally a smoother-than-normal paper. The other key factor in image quality is absorption. When the ink is sprayed onto the paper, it should stay in a tight, symmetrical dot. The ink should not be absorbed too much into the paper. If that happens, the dot will begin to feather. This means that it will spread out in an irregular fashion to cover a slightly larger area than the printer expects it to. The result is an page that looks somewhat fuzzy, particularly at the edges of objects and text. As stated, feathering is caused by the paper absorbing the ink. To combat this, high-quality inkjet paper is coated with a waxy film that keeps the ink on the surface of the paper. Coated paper normally yields a dramatically better print than other paper. The low absorption of coated paper is key to the high resolution capabilities of many of today's inkjet printers. For example, a typical Epson inkjet printer can print at a resolution of up to 720x720 dpi on standard paper. With coated paper, the resolution increases to 1440x720 dpi. The reason is that the printer can actually shift the paper slightly and add a second row of dots for every normal row, knowing that the image will not feather and cause the dots to blur together. Inkjet printers are capable of printing on a variety of media. Commercial inkjet printers sometimes spray directly on an item like the label on a beer bottle. For consumer use, there are a number of specialty papers, ranging from adhesive-backed labels or stickers to business cards and brochures. You can even get iron-on transfers that allow you to create an image and put it on a T-shirt! One thing is for certain, inkjet printers definitely provide an easy and affordable way to unleash your creativity. Refilling Cartridges Because of the expense of inkjet cartridges, a huge business has grown around the idea of refilling them. For most people, refilling makes good sense, but there are a few things to be aware of: Make sure the refill kit is for your printer model. As you learned in the previous section, different printers use different technologies for putting the ink on the paper. If the wrong type of ink is used, it can degrade the output or possibly damage the printer. While some commercial inkjets use oil- based inks, virtually all desktop inkjets for home or office use have water-based ink. The exact ink composition varies greatly between manufacturers. For example, thermal bubble inkjets need ink that is stable at higher temperatures than piezoelectric printers. Most manufacturers require that you use only their approved ink. Refill kits normally will void your warranty. While you can refill cartridges, be very careful of the ones that have the print head built into the cartridge. You do not want to refill these more than two or three times, or the print head will begin to deteriorate and could damage your printer. Check out this site for some good links and information about inkjet refills. MODEM http://computer.howstuffworks.com/modem.htm If you are reading this article on your computer at home, it probably arrived via modem. In this edition of HowStuffWorks, we'll show you how a modem brings you Web pages. We'll start with the original 300-baud modems and progress all the way through to the ADSL configurations! (Note: If you are unfamiliar with bits, bytes and the ASCII character codes, reading How Bits and Bytes Work will help make this article much clearer.) Let's get started with a short recap of how the modem came to be The Origin of Modems The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back. Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below: In a configuration like this, a dumb terminal at an off-site office or store could "dial in" to a large, central computer. The 1960s were the age of time-shared computers, so a business would often buy computer time from a time-share facility and connect to it via a 300-bit-per-second (bps) modem. A dumb terminal is simply a keyboard and a screen. A very common dumb terminal at the time was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user typed a character on the terminal, the modem sent the ASCII code for the character to the computer. The computer then sent the character back to the computer so it would appear on the screen. When personal computers started appearing in the late 1970s, bulletin board systems (BBS) became the rage. A person would set up a computer with a modem or two and some BBS software, and other people would dial in to connect to the bulletin board. The users would run terminal emulators on their computers to emulate a dumb terminal. People got along at 300 bps for quite a while. The reason this speed was tolerable was because 300 bps represents about 30 characters per second, which is a lot more characters per second than a person can type or read. Once people started transferring large programs and images to and from bulletin board systems, however, 300 bps became intolerable. Modem speeds went through a series of steps at approximately two-year intervals: 300 bps - 1960s through 1983 or so 1200 bps - Gained popularity in 1984 and 1985 2400 bps 9600 bps - First appeared in late 1990 and early 1991 19.2 kilobits per second (Kbps) 28.8 Kbps 33.6 Kbps 56 Kbps - Became the standard in 1998 ADSL, with theoretical maximum of up to 8 megabits per second (Mbps) - Gained popularity in 1999 (Check out How DSL Works and How Cable Modems Work for more information on the progression of modem technology and current speeds.) 300-bps Modems We'll use 300-bps modems as a starting point because they are extremely easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits (see How Guitars Work for a discussion of tones and frequencies). When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare. Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary (see How Bits and Bytes Work for details on binary). A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port). The terminal's modem is connected to the RS- 232 port, so it receives the bits one at a time and its job is to send them over the phone line. Faster Modems In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques (see the links at the end of this article for more information). All of these high-speed modems incorporate a concept of gradual degradation, meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed. The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central office and your house are equipped with an ADSL modem on your line, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data. The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe. Point-to-Point Protocol Today, no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP. The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer. See this page for additional information on PPP. If you want to know more about modems, protocols, and especially if you wish to delve into things like PSK and QAM in more detail, check out the links on the next page! MUS http://computer.howstuffworks.com/mouse.htm Mice first broke onto the public stage with the introduction of the Apple Macintosh in 1984, and since then they have helped to completely redefine the way we use computers. Every day of your computing life, you reach out for your mouse whenever you want to move your cursor or activate something. Your mouse senses your motion and your clicks and sends them to the computer so it can respond appropriately. In this article we'll take the cover off of this important part of the human-machine interface and see exactly what makes it tick. Evolution It is amazing how simple and effective a mouse is, and it is also amazing how long it took mice to become a part of everyday life. Given that people naturally point at things -- usually before they speak -- it is surprising that it took so long for a good pointing device to develop. Although originally conceived in the 1960s, a couple of decades passed before mice became mainstream. In the beginning, there was no need to point because computers used crude interfaces like teletype machines or punch cards for data entry. The early text terminals did nothing more than emulate a teletype (using the screen to replace paper), so it was many years (well into the 1960s and early 1970s) before arrow keys were found on most terminals. Full screen editors were the first things to take real advantage of the cursor keys, and they offered humans the first way to point. Light pens were used on a variety of machines as a pointing device for many years, and graphics tablets, joy sticks and various other devices were also popular in the 1970s. None of these really took off as the pointing device of choice, however. When the mouse hit the scene -- attached to the Mac, it was an immediate success. There is something about it that is completely natural. Compared to a graphics tablet, mice are extremely inexpensive and they take up very little desk space. In the PC world, mice took longer to gain ground, mainly because of a lack of support in the operating system. Once Windows 3.1 made Graphical User Interfaces (GUIs) a standard, the mouse became the PC-human interface of choice very quickly. Inside a Mouse The main goal of any mouse is to translate the motion of your hand into signals that the computer can use. Let's take a look inside a track-ball mouse to see how it works: A ball inside the mouse touches the desktop and rolls when the mouse moves. Two rollers inside the mouse touch the ball. One of the rollers is oriented so that it detects motion in the X direction, and the other is oriented 90 degrees to the first roller so it detects motion in the Y direction. When the ball rotates, one or both of these rollers rotate as well. The following image shows the two white rollers on this mouse: The rollers each connect to a shaft, and the shaft spins a disk with holes in it. When a roller rolls, its shaft and disk spin. The following image shows the disk: On either side of the disk there is an infrared LED and an infrared sensor. The holes in the disk break the beam of light coming from the LED so that the infrared sensor sees pulses of light. The rate of the pulsing is directly related to the speed of the mouse and the distance it travels. An on-board processor chip reads the pulses from the infrared sensors and turns them into binary data that the computer can understand. The chip sends the binary data to the computer through the mouse's cord. In this optomechanical arrangement, the disk moves mechanically, and an optical system counts pulses of light. On this mouse, the ball is 21 mm in diameter. The roller is 7 mm in diameter. The encoding disk has 36 holes. So if the mouse moves 25.4 mm (1 inch), the encoder chip detects 41 pulses of light. You might have noticed that each encoder disk has two infrared LEDs and two infrared sensors, one on each side of the disk (so there are four LED/sensor pairs inside a mouse). This arrangement allows the processor to detect the disk's direction of rotation. There is a piece of plastic with a small, precisely located hole that sits between the encoder disk and each infrared sensor. It is visible in this photo: This piece of plastic provides a window through which the infrared sensor can "see." The window on one side of the disk is located slightly higher than it is on the other -- one-half the height of one of the holes in the encoder disk, to be exact. That difference causes the two infrared sensors to see pulses of light at slightly different times. There are times when one of the sensors will see a pulse of light when the other does not, and vice versa. This page offers a nice explanation of how direction is determined. Data Interface Most mice on the market today use a USB connector to attach to your computer. USB is a standard way to connect all kinds of peripherals to your computer, including printers, digital cameras, keyboards and mice. See How USB Ports Work for more information about this technology. Some older mice, many of which are still in use today, have a PS/2 type connector, as shown here: Instead of a PS/2 connector, a few other older mice use a serial type of connector to attach to a computer. See How Serial Ports Work for more information. Optical Mice Developed by Agilent Technologies and introduced to the world in late 1999, the optical mouse actually uses a tiny camera to take thousands of pictures every second. Able to work on almost any surface without a mouse pad, most optical mice use a small, red light- emitting diode (LED) that bounces light off that surface onto a complimentary metal-oxide semiconductor (CMOS) sensor. In addition to LEDs, a recent innovation are laser-based optical mice that detect more surface details compared to LED technology. This results in the ability to use a laser-based optical mouse on even more surfaces than an LED mouse. Here's how the sensor and other parts of an optical mouse work together: The CMOS sensor sends each image to a digital signal processor (DSP) for analysis. The DSP detects patterns in the images and examines how the patterns have moved since the previous image. Based on the change in patterns over a sequence of images, the DSP determines how far the mouse has moved and sends the corresponding coordinates to the computer. The computer moves the cursor on the screen based on the coordinates received from the mouse. This happens hundreds of times each second, making the cursor appear to move very smoothly. Optical mice have several benefits over track-ball mice: No moving parts means less wear and a lower chance of failure. There's no way for dirt to get inside the mouse and interfere with the tracking sensors. Increased tracking resolution means a smoother response. They don't require a special surface, such as a mouse pad. Back to the Drawing Board Another type of optical mouse has been around for over a decade. The original optical-mouse technology bounced a focused beam of light off a highly-reflective mouse pad onto a sensor. The mouse pad had a grid of dark lines. Each time the mouse was moved, the beam of light was interrupted by the grid. Whenever the light was interrupted, the sensor sent a signal to the computer and the cursor moved a corresponding amount. This kind of optical mouse was difficult to use, requiring that you hold it at precisely the right angle to ensure that the light beam and sensor aligned. Also, damage to or loss of the mouse pad rendered the mouse useless until a replacement pad was purchased. Today's optical mice are far more user- friendly and reliable. Accuracy A number of factors affect the accuracy of an optical mouse. One of the most important aspects is resolution. The resolution is the number of pixels per inch that the optical sensor and focusing lens "see" when you move the mouse. Resolution is expressed as dots per inch (dpi). The higher the resolution, the more sensitive the mouse is and the less you need to move it to obtain a response. Most mice have a resolution of 400 or 800 dpi. However, mice designed for playing electronic games can offer as much as 1600 dpi resolution. Some gaming mice also allow you to decrease the dpi on the fly to make the mouse less sensitive in situations when you need to make smaller, slower movements. Historically, corded mice have been more responsive than wireless mice. This fact is changing, however, with the advent of improvements in wireless technologies and optical sensors. Other factors that affect quality include: Size of the optical sensor -- larger is generally better, assuming the other mouse components can handle the larger size. Sizes range from 16 x 16 pixels to 30 x 30 pixels. Refresh rate -- it is how often the sensor samples images as you move the mouse. Faster is generally better, assuming the other mouse components can process them. Rates range from 1500 to 6000 samples per second. Image processing rate -- is a combination of the size of the optical sensor and the refresh rate. Again, faster is better and rates range from 0.486 to 5.8 megapixels per second. Maximum speed -- is the maximum speed that you can move the mouse and obtain accurate tracking. Faster is better and rates range from 16 to 40 inches per second. Wireless Mice Most wireless mice use radio frequency (RF) technology to communicate information to your computer. Being radio-based, RF devices require two main components: a transmitter and a receiver. Here's how it works: The transmitter is housed in the mouse. It sends an electromagnetic (radio) signal that encodes the information about the mouse's movements and the buttons you click. The receiver, which is connected to your computer, accepts the signal, decodes it and passes it on to the mouse driver software and your computer's operating system. The receiver can be a separate device that plugs into your computer, a special card that you place in an expansion slot, or a built-in component. Many electronic devices use radio frequencies to communicate. Examples include cellular phones, wireless networks, and garage door openers. To communicate without conflicts, different types of devices have been assigned different frequencies. Newer cell phones use a frequency of 900 megahertz, garage door openers operate at a frequency of 40 megahertz, and 802.11b/g wireless networks operate at 2.4 gigahertz. Megahertz (MHz) means "one million cycles per second," so "900 megahertz" means that there are 900 million electromagnetic waves per second. Gigahertz (GHz) means "one billion cycles per second." To learn more about RF and frequencies, see How the Radio Spectrum Works. Benefits Unlike infrared technology, which is commonly used for short-range wireless communications such as television remote controls, RF devices do not need a clear line of sight between the transmitter (mouse) and receiver. Just like other types of devices that use radio waves to communicate, a wireless mouse signal can pass through barriers such as a desk or your monitor. RF technology provides a number of additional benefits for wireless mice. These include: RF transmitters require low power and can run on batteries RF components are inexpensive RF components are light weight As with most mice on the market today, wireless mice use optical sensor technology rather than the earlier track-ball system. Optical technology improves accuracy and lets you use the wireless mouse on almost any surface -- an important feature when you're not tied to your computer by a cord. Pairing and Security In order for the transmitter in the mouse to communicate with its receiver, they must be paired. This means that both devices are operating at the same frequency on the same channel using a common identification code. A channel is simply a specific frequency and code. The purpose of pairing is to filter out interference from other sources and RF devices. Pairing methods vary, depending on the mouse manufacturer. Some devices come pre-paired. Others use methods such as a pairing sequence that occurs automatically, when you push specific buttons, or when you turn a dial on the receiver and/or mouse. To protect the information your mouse transmits to the receiver, most wireless mice include an encryption scheme to encode data into an unreadable format. Some devices also use a frequency hopping method, which causes the mouse and receiver to automatically change frequencies using a predetermined pattern. This provides additional protection from interference and eavesdropping. Bluetooth Mice One of the RF technologies that wireless mice commonly use is Bluetooth. Bluetooth technology wirelessly connects peripherals such as printers, headsets, keyboards and mice to Bluetooth-enabled devices such as computers and personal digital assistants (PDAs). Because a Bluetooth receiver can accommodate multiple Bluetooth peripherals at one time, Bluetooth is also known as a personal area network (PAN). Bluetooth devices have a range of about 33 feet (10 meters). Bluetooth operates in the 2.4 GHz range using RF technology. It avoids interference among multiple Bluetooth peripherals through a technique called spread-spectrum frequency hopping. WiFi devices such as 802.11b/g wireless networks also operate in the 2.4 GHz range, as do some cordless telephonescordless telephones and microwave ovens. Version 1.2 of Bluetooth provides adaptive frequency hopping (AFH), which is an enhanced frequency-hopping technology designed to avoid interference with other 2.4 GHz communications. Why is it called Bluetooth? Harald Bluetooth was king of Denmark in the late 900s. He managed to unite Denmark and part of Norway into a single kingdom then introduced Christianity into Denmark. He left a large monument, the Jelling rune stone, in memory of his parents. He was killed in 986 during a battle with his son, Svend Forkbeard. Choosing this name for the standard indicates how important companies from the Baltic region (nations including Denmark, Sweden, Norway and Finland) are to the communications industry, even if it says little about the way the technology works. RF Mice The other common type of wireless mouse is an RF device that operates at 27 MHz and has a range of about 6 feet (2 meters). More recently, 2.4 GHz RF mice have hit the market with the advantage of a longer range -- about 33 feet (10 meters) and faster transmissions with less interference. Multiple RF mice in one room can result in cross-talk, which means that the receiver inadvertently picks up the transmissions from the wrong mouse. Pairing and multiple channels help to avoid this problem. Typically, the RF receiver plugs into a USB port and does not accept any peripherals other than the mouse (and perhaps a keyboard, if sold with the mouse). Some portable models designed for use with notebook computers come with a compact receiver that can be stored in a slot inside the mouse when not in use. Mouse Tip If you want to use both a wireless RF mouse and keyboard, buy them together. Pairing and transmission technology is unique to each manufacturer and device. If you purchase an RF wireless keyboard and mouse separately, you may have to connect a receiver for each one to your PC. Working Together Some PC keyboards and mice are designed to work together to give you more options for input. For example, the Logitech Cordless Desktop LX700 comes with a keyboard that has scroll, pan and zoom capabilities. The mouse includes the same features, so that you can use either to perform these functions. Multi-Media Mouse and Combination Mouse/Remote These types of mice are used with multimedia systems such as the Windows XP Media Center Edition computers. Some combine features of a mouse with additional buttons (such as play, pause, forward, back and volume) for controlling media. Others resemble a television/media player remote control with added features for mousing. Remote controls generally use infrared sensors but some use a combination of infrared and RF technology for greater range. Gaming Mice Gaming mice are high-precision, optical mice designed for use with PCs and game controllers. Features may include: Multiple buttons for added flexibility and functions such as adjusting dpi rates on the fly Wireless connectivity and an optical sensor Motion feedback and two-way communication Motion-Based Mice Yet another innovation in mouse technology is motion-based control. With this feature, you control the mouse pointer by waving the mouse in the air. The technology patented by one manufacturer, Gyration, incorporates miniature gyroscopes to track the motion of the mouse as you wave it in the air. It uses an electromagnetic transducer and sensors to detect rotation in two axes at the same time. The mouse operates on the principle of the Coriolis Effect, which is the apparent turning of an object that's moving in relation to another rotating object. The device and accompanying software converts the mouse movements into movements on the computer's screen. The mice also include an optical sensor for use on a desktop. Biometric Mice Biometric mice add security to your computer system by permitting only authorized users to control the mouse and access the computer. Protection is accomplished with an integrated fingerprint reader either in the receiver or the mouse. This feature enhances security and adds convenience because you can use your fingerprint rather than passwords for a secure login. To use the biometric feature, a software program that comes with the mouse registers fingerprints and stores information about corresponding authorized users. Some software programs also let you encrypt and decrypt files. For more information about biometric fingerprint technology, see How Fingerprint Scanners Work. Tilting Scroll Wheel A recent innovation in mouse scrolling is a tilting scroll wheel that allows you to scroll onscreen both horizontally (left/right) and vertically (up/down). The ability to scroll both ways is handy when you are viewing wide documents like a Web page or spreadsheet. To navigate both horizontally and vertically, the scroll wheel is positioned on a combination fulcrum and lever. This is the design used by the Logitech Cordless Click! Plus mouse. Another method for vertical and horizontal scrolling is a touch scroll panel that responds to your finger sliding horizontally and vertically, as employed by the Logitech V500 Cordless Notebook Mouse. PLUG AND PLAY http://www.pcguide.com/ref/mbsys/res/pnp-c.html The large variety of different cards that can be added to PCs to expand their capabilities is both a blessing and a curse. As you can see from the other sections that have discussed system resources, configuring the system and dealing with resource conflicts is part of the curse of having so many different non-standard devices on the market. Dealing with these issues can be a tremendously confusing, difficult and time-consuming task. In fact, many users have stated that this is the single most frustrating part of owning and maintaining a PC, or of upgrading the PC's hardware. In an attempt to resolve this ongoing problem, the Plug and Play (also called PnP) specification was developed by Microsoft with cooperation from Intel and many other hardware manufacturers. The goal of Plug and Play is to create a computer whose hardware and software work together to automatically configure devices and assign resources, to allow for hardware changes and additions without the need for large-scale resource assignment tweaking. As the name suggests, the goal is to be able to just plug in a new device and immediately be able to use it, without complicated setup maneuvers. A form of Plug and Play was actually first made available on the EISA and MCA buses many years ago. For several reasons, however, neither of these buses caught on and became popular. PnP hit the mainstream in 1995 with the release of Windows 95 and PC hardware designed to work with it. Requirements for Plug and Play Automatically detecting and configuring hardware and software is not a simple task. To perform this work, cooperation is required from several hardware and software areas. The four "partners" that must be Plug and Play compliant in order for it to work properly are: System Hardware: The hardware on your system, through the system chipset and system bus controllers, must be capable of handling PnP devices. For modern PCI-based systems this is built in, as PCI was designed with PnP in mind. Most PCI-based systems also support PnP on their ISA bus, with special circuitry to link the two together and share resource information. Older PCs with ISA-only or VL-bus system buses generally do not support Plug and Play. Peripheral Hardware: The devices that you are adding into the system must themselves be PnP compatible. PnP is now supported for a wide variety of devices, from modems and network cards inside the box to printers and even monitors outside it. These devices must be PnP-aware so that they are capable of identifying themselves when requested, and able to accept resource assignments from the system when they are made. The System BIOS: The system BIOS plays a key role in making Plug and Play work. Routines built into the BIOS perform the actual work of collecting information about the different devices and determining what should use which resources. The BIOS also communicates this information to the operating system, which uses it to configure its drivers and other software to make the devices work correctly. In many cases older PCs that have an outdated BIOS but otherwise have support for PnP in hardware (PCI-based Pentiums produced between 1993 and 1995 are the prime candidates) can be made PnP-compliant through a BIOS upgrade. The Operating System: Finally, the operating system must be designed to work with the BIOS (and thus indirectly, with the hardware as well). The operating system sets up any low-level software (such as device drivers) that are necessary for the device to be used by applications. It also communicates with the user, notifying him or her of changes to the configuration, and allows changes to be made to resource settings if necessary. Currently, the only mainstream operating system with full PnP support is Windows 95. As you can see, you need a lot for Plug and Play to work, and this is why the vast majority of older systems (pre-1996) do not properly support this standard. Plug and Play Operation Most of the actual work involved in making Plug and Play function is performed by the system BIOS during the boot process. At the appropriate step of the boot process, the BIOS will follow a special procedure to determine and configure the Plug and Play devices in your system. Here is a rough layout of the steps that the BIOS follows at boot time when managing a PCI-based Plug and Play system: Create a resource table of the available IRQs, DMA channels and I/O addresses, excluding any that are reserved for system devices. Search for and identify PnP and non-PnP devices on the PCI and ISA buses. Load the last known system configuration from the ESCD area stored in non-volatile memory. Compare the current configuration to the last known configuration. If they are unchanged, continue with the boot; this part of the boot process ends and the rest of the bootup continues from here. If the configuration is new, begin system reconfiguration. Start with the resource table by eliminating any resources being used by non-PnP devices. Check the BIOS settings to see if any additional system resources have been reserved for use by non-PnP devices and eliminate any of these from the resource table. Assign resources to PnP cards from the resources remaining in the resource table, and inform the devices of their new assignments. Update the ESCD area by saving to it the new system configuration. Most BIOSes will print a message when this happens like "Updating ESCD ... Successful". Continue with the boot. Tip: See the section on PCI / PnP in the BIOS area, which describes the BIOS settings that affect how PnP works in a PCI system. Extended System Configuration Data (ESCD) If the BIOS were to assign resources to each PnP device on every boot, two problems would result. First, it would take time to do something that it has already done before, each boot, for no purpose. After all, most people change their system hardware relatively infrequently. Second and more importantly, it is possible that the BIOS might not always make the same decision when deciding how to allocate resources, and you might find them changing even when the hardware remains unchanged. ESCD is designed to overcome these problems. The ESCD area is a special part of your BIOS's CMOS memory, where BIOS settings are held. This area of memory is used to hold configuration information for the hardware in your system. At boot time the BIOS checks this area of memory and if no changes have occurred since the last bootup, it knows it doesn't need to configure anything and skips that portion of the boot process. ESCD is also used as a communications link between the BIOS and the operating system. Both use the ESCD area to read the current status of the hardware and to record changes. Windows 95 reads the ESCD to see if hardware has been changed and react accordingly. Windows 95 also allows users to override Plug and Play resource assignments by manually changing resources in the Device Manager. This information is recorded in the ESCD area so the BIOS knows about the change at the next boot and doesn't try to change the assignment back again. The ESCD information is stored in a non-volatile CMOS memory area, the same way that standard BIOS settings are stored. Note: Some (relatively rare) systems using Windows 95 can exhibit strange behavior that is caused by incompatibility between how Windows 95 and the BIOS are using ESCD. This can cause an "Updating ESCD" message to appear each and every time the system is booted, instead of only when the hardware is changed. See here for more details. Plug and Play and Non-Plug-and-Play Devices Devices that do not support the PnP standard can be used in a PnP system, but they present special problems. These are called legacy devices, which is geekspeak for "old hardware we have to keep using even though it doesn't have the capabilities we wish it did". :^) They make resource assignment much more difficult because they cannot be automatically configured by the BIOS. Generally, the BIOS deals with non-PnP devices by ignoring them. It simply considers them as "part of the scenery" and avoids any resources they are using. There is usually no problem using these devices with PnP, but using too many non-PnP devices can make it more difficult for PnP to work, due to the large number of resources that it is not allowed to touch. "Plug and Pray" :^) This amusing sarcastic name for Plug and Play has become all too commonly heard these days. It refers to the large number of problems associated with getting Plug and Play to work on many systems. It's odd to consider--wasn't the whole point of Plug and Play to make it easier to configure systems? It is, but unfortunately PnP falls short of its lofty goal in many cases. When you use PnP, you are essentially turning over control of system configuration to the PC. The problem is a common one in computers: the computer isn't as smart as the human, or more specifically, the computer isn't as "resourceful" (no pun intended. :^) ). Computers are not nearly as good as humans at realizing things like this: "Well, if I put the modem at that value and the printer there, I will have a conflict. But I can fix that by changing the assignment for the sound card, moving the modem over here, and putting the printer there". The system can take care of the simple situations, but can become confused by more complicated ones. The use of multiple "legacy" ISA devices can exacerbate this. Generally, the more complex your setup, the more likely you will need to manual "tweak" whatever PnP comes up with by default. The biggest problems with Plug and Play revolve around its apparent "stubbornness". At times, the BIOS and operating system seem determined to put a device at a location where you do not want it. For example, you may have a modem that you want at COM3 and IRQ5, but the BIOS may decide to put it at COM4 and IRQ3, conflicting with the COM2 serial port. This can get quite aggravating to deal with. Also, some people just prefer the feeling of being "in control" that they lose when PnP is used. (I must admit to being one of these people, oftentimes.) The problems with PnP are less common now than they were in the first year that it was announced. As with any new technology--especially one that is as complex as PnP and that involves so many parts of the system--it takes time to iron the bugs out. Most systems today work quite well with PnP. In most cases problems with PnP are due to incorrect system configuration, manual overrides of PnP devices through the Windows 95 Device Manager, or incorrect BIOS settings. TASTATUR http://computer.howstuffworks.com/keyboard.htm The part of the computer that we come into most contact with is probably the piece that we think about the least. But the keyboard is an amazing piece of technology. For instance, did you know that the keyboard on a typical computer system is actually a computer itself? At its essence, a keyboard is a series of switches connected to a microprocessor that monitors the state of each switch and initiates a specific response to a change in that state. In this edition of How Stuff Works, you will learn more about this switching action, and about the different types of keyboards, how they connect and talk to your computer, and what the components of a keyboard are. Types of Keyboards Keyboards have changed very little in layout since their introduction. In fact, the most common change has simply been the natural evolution of adding more keys that provide additional functionality. The most common keyboards are: 101-key Enhanced keyboard 104-key Windows keyboard 82-key Apple standard keyboard 108-key Apple Extended keyboard Portable computers such as laptops quite often have custom keyboards that have slightly different key arrangements than a standard keyboard. Also, many system manufacturers add specialty buttons to the standard layout. A typical keyboard has four basic types of keys: Typing keys Numeric keypad Function keys Control keys The typing keys are the section of the keyboard that contain the letter keys, generally laid out in the same style that was common for typewriters. This layout, known as QWERTY for the first six letters in the layout, was originally designed to slow down fast typists by making the arrangement of the keys somewhat awkward! The reason that typewriter manufacturers did this was because the mechanical arms that imprinted each character on the paper could jam together if the keys were pressed too rapidly. Because it has been long established as a standard, and people have become accustomed to the QWERTY configuration, manufacturers developed keyboards for computers using the same layout, even though jamming is no longer an issue. Critics of the QWERTY layout have adopted another layout, Dvorak, that places the most commonly used letters in the most convenient arrangement. The numeric keypad is a part of the natural evolution mentioned previously. As the use of computers in business environments increased, so did the need for speedy data entry. Since a large part of the data was numbers, a set of 17 keys was added to the keyboard. These keys are laid out in the same configuration used by most adding machines and calculators, to facilitate the transition to computer for clerks accustomed to these other machines. In 1986, IBM extended the basic keyboard with the addition of function and control keys. The function keys, arranged in a line across the top of the keyboard, could be assigned specific commands by the current application or the operating system. Control keys provided cursor and screen control. Four keys arranged in an inverted T formation between the typing keys and numeric keypad allow the user to move the cursor on the display in small increments. The control keys allow the user to make large jumps in most applications. Common control keys include: Home End Insert Delete Page Up Page Down Control (Ctrl) Alternate (Alt) Escape (Esc) The Windows keyboard adds some extra control keys: two Windows or Start keys, and an Application key. The Apple keyboards are specific to Apple Mac systems. Inside the Keyboard The processor in a keyboard has to understand several things that are important to the utility of the keyboard, such as: Position of the key in the key matrix. The amount of bounce and how to filter it. The speed at which to transmit the typematics. The key matrix is the grid of circuits underneath the keys. In all keyboards except for capacitive ones, each circuit is broken at the point below a specific key. Pressing the key bridges the gap in the circuit, allowing a tiny amount of current to flow through. The processor monitors the key matrix for signs of continuity at any point on the grid. When it finds a circuit that is closed, it compares the location of that circuit on the key matrix to the character map in its ROM. The character map is basically a comparison chart for the processor that tells it what the key at x,y coordinates in the key matrix represents. If more than one key is pressed at the same time, the processor checks to see if that combination of keys has a designation in the character map. For example, pressing the a key by itself would result in a small letter "a" being sent to the computer. If you press and hold down the Shift key while pressing the a key, the processor compares that combination with the character map and produces a capital letter "A." The character map in the keyboard can be superseded by a different character map provided by the computer. This is done quite often in languages whose characters do not have English equivalents. Also, there are utilities for changing the character map from the traditional QWERTY to DVORAK or another custom version. Keyboards rely on switches that cause a change in the current flowing through the circuits in the keyboard. When the key presses the keyswitch against the circuit, there is usually a small amount of vibration between the surfaces, known as bounce. The processor in a keyboard recognizes that this very rapid switching on and off is not caused by you pressing the key repeatedly. Therefore, it filters all of the tiny fluctuations out of the signal and treats it as a single keypress. If you continue to hold down a key, the processor determines that you wish to send that character repeatedly to the computer. This is known as typematics. In this process, the delay between each instance of a character can normally be set in software, typically ranging from 30 characters per second (cps) to as few as two cps. Keyboard Technologies Keyboards use a variety of switch technologies. It is interesting to note that we generally like to have some audible and tactile response to our typing on a keyboard. We want to hear the keys "click" as we type, and we want the keys to feel firm and spring back quickly as we press them. Let's take a look at these different technologies: Rubber dome mechanical Capacitive non-mechanical Metal contact mechanical Membrane mechanical Foam element mechanical Probably the most popular switch technology in use today is rubber dome. In these keyboards, each key sits over a small, flexible rubber dome with a hard carbon center. When the key is pressed, a plunger on the bottom of the key pushes down against the dome. This causes the carbon center to push down also, until it presses against a hard flat surface beneath the key matrix. As long as the key is held, the carbon center completes the circuit for that portion of the matrix. When the key is released, the rubber dome springs back to its original shape, forcing the key back up to its at-rest position. Rubber dome switch keyboards are inexpensive, have pretty good tactile response and are fairly resistant to spills and corrosion because of the rubber layer covering the key matrix. Membrane switches are very similar in operation to rubber dome keyboards. A membrane keyboard does not have separate keys though. Instead, it has a single rubber sheet with bulges for each key. You have seen membrane switches on many devices designed for heavy industrial use or extreme conditions. Because they offer almost no tactile response and can be somewhat difficult to manipulate, these keyboards are seldom found on normal computer systems. Capacitive switches are considered to be non-mechanical because they do not simply complete a circuit like the other keyboard technologies. Instead, current is constantly flowing through all parts of the key matrix. Each key is spring-loaded, and has a tiny plate attached to the bottom of the plunger. When a key is pressed, this plate is brought very close to another plate just below it. As the two plates are brought closer together, it affects the amount of current flowing through the matrix at that point. The processor detects the change and interprets it as a keypress for that location. Capacitive switch keyboards are expensive, but do not suffer from corrosion and have a longer life than any other keyboard. Also, they do not have problems with bounce since the two surfaces never come into actual contact. Metal contact and foam element keyboards are not as common as they used to be. Metal contact switches simply have a spring-loaded key with a strip of metal on the bottom of the plunger. When the key is pressed, the metal strip connects the two parts of the circuit. The foam element switch is basically the same design but with a small piece of spongy foam between the bottom of the plunger and the metal strip, providing for a better tactile response. Both technologies have good tactile response, make satisfyingly audible "clicks" and are inexpensive to produce. The problem is that the contacts tend to wear out or corrode faster than on keyboards that use other technologies. Also, there is no barrier that prevents dust or liquids from coming in direct contact with the circuitry of the key matrix. From the Keyboard to the Computer As you type, the processor in the keyboard is analyzing the key matrix and determining what characters to send to the computer. It maintains these characters in a buffer of memory that is usually about 16 bytes large. It then sends the data in a stream to the computer via some type of connection. The most common keyboard connectors are: 5-pin DIN (Deustche Industrie Norm) connector 6-pin IBM PS/2 mini-DIN connector 4-pin USB (Universal Serial Bus) connector internal connector (for laptops) Normal DIN connectors are rarely used anymore. Most computers use the mini-DIN PS/2 connector; but an increasing number of new systems are dropping the PS/2 connectors in favor of USB. No matter which type of connector is used, two principal elements are sent through the connecting cable. The first is power for the keyboard. Keyboards require a small amount of power, typically about 5 volts, in order to function. The cable also carries the data from the keyboard to the computer. The other end of the cable connects to a port that is monitored by the computer's keyboard controller. This is an integrated circuit (IC) whose job is to process all of the data that comes from the keyboard and forward it to the operating system. When the operating system is notified that there is data from the keyboard, a number of things can happen: It checks to see if the keyboard data is a system level command. A good example of this is Ctrl-Alt- Delete on a Windows computer, which initiates a reboot. The operating system then passes the keyboard data on to the current application. The current application understands the keyboard data as an application-level command. An example of this would be Alt - f, which opens the File menu in a Windows application. The current application is able to accept keyboard data as content for the application (anything from typing a document to entering a URL to performing a calculation), or The current application does not accept keyboard data and therefore ignores the information. Once the keyboard data is identified as either system-specific or application-specific, it is processed accordingly. The really amazing thing is how quickly all of this happens. As I type this article, there is no perceptible time lapse between my fingers pressing the keys and the characters appearing on my monitor. When you think about everything the computer is doing to make each single character appear, it is simply incredible! DATABUS http://www.dbcsoftware.com/dbcov.html DB/C DX, DATABUS, and PL/B Overview DB/C DX is a program development tool for the DATABUS programming language. DB/C DX includes the compiler, the run-time executive and eighteen utilities. The utilities provide functions such as file management, file sorting, file indexing, library management, source file editing and more. DB/C DX is available for a variety of different computer operating systems including Windows 95 through XP based personal computers, LINUX, most UNIX computer systems, and Apple Mac OS X. What is DATABUS? DATABUS is a high level computer language designed for writing business oriented applications. In some respects DATABUS is like COBOL, although DATABUS contains several sophisticated features that are not available in COBOL or in other business languages. DATABUS is used to create highly interactive applications that contain friendly user interfaces. DATABUS is also used to create processing programs that deal with the large data files typically found in business applications. DATABUS was created by Datapoint Corporation in the early 1970s. Until 1981, Datapoint was the only company providing a DATABUS compiler. Since then, at least six other companies have written and are currently marketing compilers for the DATABUS language. DATABUS was accepted as an ANSI standard in December 1994. In the process it was given the name PL/B because Datapoint refused to relinquish its trademark on the name DATABUS. People still generally refer to it as DATABUS. Why use DATABUS? DATABUS has always been a language that is easy to learn and use. Other languages that offer these benefits typically have few operations and limit the functions available to the programmer. DATABUS is easy to use because of its structure and readability. The syntax is English-like and there are no cryptic characters to remember. But don't let this fool you—DATABUS contains over 125 separate operations (called verbs) that provide the competent programmer with an arsenal of functions. Here is an example of typical DATABUS code: . . THIS PL/B CODE FRAGMENT WILL LOOK UP THE . TELEPHONE NUMBER OF AN EMPLOYEE BY EMPLOYEE NUMBER . LOOP KEYIN "ENTER AN EMPLOYEE NUMBER: ", EMPNUM STOP IF F3 READ EMPLOYEE, EMPNUM; NAME, TELNUM IF OVER BEEP DISPLAY "EMPLOYEE NUMBER NOT ON FILE" ELSE DISPLAY "NAME: ", NAME, "TELEPHONE: ", TELNUM ENDIF REPEAT Many business languages in use today were really designed for mainframe batch operation or single user PC operation. The aspect of multiple users accessing common files interactively is only an add-on in these languages. However, DATABUS was designed from the beginning to be run in an interactive, multi-user environment. The functions available to the programmer for screen display and keyboard handling are excellent. The data access and locking mechanisms are time tested and stand up well in a high performance, heavy usage operation. DATABUS is also a fine complement to SQL based database systems. Even though most SQL database systems come with a built-in 4GL, many database applications are still being written in a third generation language. The reasons for this vary, but the bottom line is that a 4GL is not capable of providing a programmer with all the functions that a third generation language provides. When the choice comes down to COBOL, C/C++, Java, VB, or DATABUS, many developers are choosing DATABUS. Here are some of the many reasons why development in DATABUS is superior: 1. Compilation is extremely fast and there is no link step at all. 2. Debugging a DATABUS program is made much easier by the fact that the language is completely closed. All variables are automatically initialized. A numeric variable cannot contain or be assigned an invalid value. There are no pointers that can be pointing into odd places causing subtle and hard to find bugs. There is no data overlaying which can cause data type mismatches. A DATABUS program cannot cause a memory dump - it's just not possible. 3. In addition to an indexed sequential access method, DATABUS provides another access method called the associative index method. Commonly called AIM, this access method allows context-free key searches into data files. For example, in a parts inventory file, it is possible to retreive all records that contain the word "BOLT" anywhere in the description field. The word may be in upper case, lower case or mixed case. The programmer does not need to pre-progam or extract keywords before the lookup - the AIM search method does it all for him. 4. The keyboard input and screen display verbs provide many more functions than corresponding functions in other languages. Pop up window display is almost trivial to implement. Display attributes such as reverse video, underline, blink, and colors are specified in the DISPLAY verb with short, easy to remember codes. The programmer does not have to look at a different section of program (or even at a separate screen map module as in certain other languages) to figure out what is displayed on the screen. It's all right in the program. Why should I choose DB/C? DB/C DX implements all aspects of the PL/B standard. DB/C DX also includes utilities that provide all of the necessary operating system level functions used in conjunction with DATABUS programs. The most important feature to understand about DB/C DX is its portability. No other language in existence today provides better portability than DB/C DX. The reason we can make this statement is simple: Programs compiled under DB/C DX can be run on any supported computer without recompilation. This level of portability provides you with an unprecedented ability to run your applications software on almost any computer you choose - with the guarantee that it will run correctly without any program changes or other programmer intervention. If you currently have DATABUS programs written for Datapoint's RMS DATABUS or DOS DATASHARE systems, porting to DB/C DX is quick and simple. Certain features of DB/C make the conversion from the Datapoint dialects of DATABUS easier. Using DB/C DX, development and testing of new or existing DATABUS programs is noticeably enhanced from what is available with other DATABUS compilers. Compilation speed is typically hundreds of thousands of lines per minute. Coupled with the fact that there is no link step, total compilation time is faster than any other general-use compiled language in existence. An entire system such as an order entry system consisting of 50 programs can be compiled in less than a minute on a Pentium based computer. On more expensive UNIX systems, compilation is even faster. Is choosing DB/C DX prudent? Yes. DB/C DX has been a very successful choice for many companies. Version 1 was first installed in 1983 on single user IBM PCs. Since then many additional major upgrades have been released that have improved DB/C DX in numerous ways. DB/C DX is currently installed in over 3000 companies in 30 countries. Here is a partial list of some of the more well known customers: Boeing Corp. Chase Manhatten Bank Computer Sciences Corporation Credit Lyonnais Bank EDS Guardian Industries Holiday Inn Hyatt Hotels Lincoln Center for Performing Arts Manufacturer's Hanover Bank Marathon Oil Company Nissan Motor Corporation Proctor & Gamble Royal Caribbean Cruise Lines Scott Paper Company State of California U.S. Army USCO Distribution Volvo DATASKJERM http://computer.howstuffworks.com/monitor.htm Because we use them daily, many of us have a lot of questions about our monitors and may not even realize it. What does "aspect ratio" mean? What is dot pitch? How much power does a display use? What is the difference between CRT and LCD? What does "refresh rate" mean? In this article, HowStuffWorks will answer all of these questions and many more. By the end of the article, you will be able to understand your current display and also make better decisions when purchasing your next one. Display Technology Often referred to as a monitor when packaged in a separate case, the display is the most-used output device on a computer. The display provides instant feedback by showing you text and graphic images as you work or play. Most desktop displays use liquid crystal display (LCD) or cathode ray tube (CRT) technology, while nearly all portable computing devices such as laptops incorporate LCD technology. Because of their slimmer design and lower energy consumption, monitors using LCD technology (also called flat panel or flat screen displays) are replacing the venerable CRT on most desktops. Standards and Resolution Resolution refers to the number of individual dots of color, known as pixels, contained on a display. Resolution is expressed by identifying the number of pixels on the horizontal axis (rows) and the number on the vertical axis (columns), such as 800x600. Resolution is affected by a number of factors, including the size of the screen. As monitor sizes have increased over the years, display standards and resolutions have changed. In addition, some manufacturers offer widescreen displays designed for viewing DVD movies. Common Display Standards and Resolutions Standard Resolution Typical Use XGA (Extended Graphics Array) 1024x768 15- and 17-inch CRT monitors 15-inch LCD monitors SXGA (Super XGA) 1280x1024 15- and 17-inch CRT monitors 17-and 19-inch LCD monitors UXGA (Ultra XGA) 1600x1200 19-, 20-, 21-inch CRT monitors 20-inch LCD monitors QXGA (Quad XGA) 2048x1536 21-inch and larger CRT monitors WXGA (Wide XGA) 1280x800 Wide aspect 15.4-inch laptops LCD displays WSXGA+ (Wide SXGA plus) 1680x1050 Wide aspect 20-inch LCD monitors WUXGA (Wide Ultra XGA) 1920x1200 Wide aspect 22-inch and larger LCD monitors In addition to the screen size, display standards and resolutions are related to something called the aspect ratio. Next, we'll discuss what an aspect ratio is and how screen size is measured. Aspect Ratio and Viewable Area Two measures describe the size of your display: the aspect ratio and the screen size. Historically, computer displays, like most televisions, have had an aspect ratio of 4:3. This means that the ratio of the width of the display screen to the height is 4 to 3. For widescreen LCD monitors, the aspect ratio is 16:9 (or sometimes 16:10 or 15:9). Widescreen LCD displays are useful for viewing DVD movies in widescreen format, playing games and displaying multiple windows side by side. High definition television (HDTV) also uses a widescreen aspect ratio. All types of displays include a projection surface, commonly referred to as the screen. Screen sizes are normally measured in inches from one corner to the corner diagonally across from it. This diagonal measuring system actually came about because the early television manufacturers wanted to make the screen size of their TVs sound more impressive. Interestingly, the way in which the screen size is measured for CRT and LCD monitors is different. For CRT monitors, screen size is measured diagonally from outside edges of the display casing. In other words, the exterior casing is included in the measurement as seen below. For LCD monitors, screen size is measured diagonally from the inside of the beveled edge. The measurement does not include the casing as indicated in the image below. Because of the differences in how CRT and LCD monitors are measured, a 17-inch LCD display is comparable to a 19-inch CRT display. For a more accurate representation of a CRT's size, find out its viewable screen size. This is the measurement of a CRT display without its outside casing. Popular screen sizes are 15, 17, 19 and 21 inches. Notebook screen sizes are smaller, typically ranging from 12 to 17 inches. As technologies improve in both desktop and notebook displays, even larger screen sizes are becoming available. For professional applications, such as medical imaging or public information displays, some LCD monitors are 40 inches or larger! Obviously, the size of the display directly affects resolution. The same pixel resolution is sharper on a smaller monitor and fuzzier on a larger monitor because the same number of pixels is spread out over a larger number of inches. An image on a 21-inch monitor with an 800x600 resolution will not appear nearly as sharp as it would on a 15-inch display at 800x600. Multi-scanning Monitors If you have been around computers for more than a decade, then you probably remember when NEC announced the MultiSync monitor. Up to that point, most monitors only understood one frequency, which meant that the monitor operated at a single fixed resolution and refresh rate. You had to match your monitor with a graphics adapter that provided that exact signal or it wouldn't work. The introduction of NEC MultiSync technology started a trend towards multi-scanning monitors. This technology allows a monitor to understand any frequency sent to it within a certain bandwidth. The benefit of a multi-scanning monitor is that you can change resolutions and refresh rates without having to purchase and install a new graphics adapter or monitor each time. Connections To display information on a monitor, your computer sends the monitor a signal. The signal can be in analog or digital format. Analog (VGA) Connection Because most CRT monitors require the signal information in analog (continuous electrical signals or waves) form and not digital (pulses equivalent to the binary digits 0 and 1), they typically use an analog connection. However, computers work in a digital world. The computer and video adapter convert digital data into analog format. A video adapter is an expansion card or component that provides the ability to convert display information into a signal that is sent to the monitor. It can also be called a graphics adapter, video card or graphics card. Once the display information is in analog form, it is sent to the monitor through a VGA cable. The cable connects at the back of the computer to an analog connector (also known as a D-Sub connector) that has 15 pins in three rows. See the diagram below: 1: Red out 6: Red return (ground) 11: Monitor ID 0 in 2: Green out 7: Green return (ground) 12: Monitor ID 1 in or data from display 3: Blue out 8: Blue return (ground) 13: Horizontal Sync out 4: Unused 9: Unused 14: Vertical Sync 5: Ground 10: Sync return (ground) 15: Monitor ID 3 in or data clock You can see that a VGA connector like this has three separate lines for the red, green and blue color signals, and two lines for horizontal and vertical sync signals. In a normal television, all of these signals are combined into a single composite video signal. The separation of the signals is one reason why a computer monitor can have so many more pixels than a TV set. Because a VGA (analog) connector does not support the use of digital monitors, the Digital Video Interface (DVI) standard was developed. DVI Connection DVI keeps data in digital form from the computer to the monitor. There's no need to convert data from digital information to analog information. LCD monitors work in a digital mode and support the DVI format. (Although, some also accept analog information, which is then converted to digital format.) At one time, a digital signal offered better image quality compared to analog technology. However, analog signal processing technology has improved over the years and the difference in quality is now minimal. The DVI specification is based on Silicon Image's Transition Minimized Differential Signaling (TMDS) and provides a high-speed digital interface. A transmitter on the video adapter sends the digital information to a receiver in the monitor. TMDS takes the signal from the video adapter, determines the resolution and refresh rate that the monitor is using, and spreads the signal out over the available bandwidth to optimize the data transfer from computer to monitor. DVI cables can be a single link cable that uses one TMDS transmitter or a dual link cable with two transmitters. A single link DVI cable and connection supports a 1920x1080 image, and a dual link cable/connection supports up to a 2048x1536 image. There are two main types of DVI connections: DVI-digital (DVI-D) is a digital-only format. It requires a video adapter with a DVI-D connection and a monitor with a DVI-D input. The connector contains 24 pins/receptacles in 3 rows of 8 plus a grounding slot for dual-link support. For single-link support, the connector contains 18 pins/receptacles. DVI-integrated (DVI-I) supports both digital and analog transmissions. This gives you the option to connect a monitor that accepts digital input or analog input. In addition to the pins/receptacles found on the DVI-D connector for digital support, a DVI-I connector has 4 additional pins/receptacles to carry an analog signal. DVI-D connectors carry a digital-only signal and DVI-I adds four pins for analog capability. Both connectors can be used with a single-link or a dual-link cable, depending upon the requirements of the display. If you buy a monitor with only a DVI (digital) connection, make sure that you have a video adapter with a DVI-D or DVI-I connection. If your video adapter has only an analog (VGA) connection, look for a monitor that supports the analog format. Color Depth The combination of the display modes supported by your graphics adapter and the color capability of your monitor determine how many colors it displays. For example, a display that operates in SuperVGA (SVGA) mode can display up to 16,777,216 (usually rounded to 16.8 million) colors because it can process a 24-bit-long description of a pixel. The number of bits used to describe a pixel is known as its bit depth. With a 24-bit bit depth, eight bits are dedicated to each of the three additive primary colors -- red, green and blue. This bit depth is also called true color because it can produce the 10,000,000 colors discernible to the human eye, while a 16-bit display is only capable of producing 65,536 colors. Displays jumped from 16-bit color to 24-bit color because working in eight-bit increments makes things a whole lot easier for developers and programmers. Simply put, color bit depth refers to the number of bits used to describe the color of a single pixel. The bit depth determines the number of colors that can be displayed at one time. Take a look at the following chart to see the number of colors different bit depths can produce: Bit-Depth Number of Colors 12 (monochrome) 24 (CGA) 4 16 (EGA) 8 256 (VGA) 16 65,536 (High Color, XGA) 24 16,777,216 (True Color, SVGA) 32 16,777,216 (True Color + Alpha Channel) Notice that the last entry in the chart is for 32 bits. This is a special graphics mode used by digital video, animation and video games to achieve certain effects. Essentially, 24 bits are used for color and the other eight bits are used as a separate layer for representing levels of translucency in an object or image. Nearly every monitor sold today can handle 24-bit color using a standard VGA connector. To create a single colored pixel, an LCD display uses three subpixels with red, green and blue filters. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue). Now that you have a general idea of the technology behind computer monitors, let's take a closer look at LCD monitors, CRT monitors, and the general buying considerations for both. LCD Monitors The Basics Liquid crystal display technology works by blocking light. Specifically, an LCD is made of two pieces of polarized glass (also called substrate) that contain a liquid crystal material between them. A backlight creates light that passes through the first substrate. At the same time, electrical currents cause the liquid crystal molecules to align to allow varying levels of light to pass through to the second substrate and create the colors and images that you see. Active and Passive Matrix Displays Most LCD displays use active matrix technology. A thin film transistor (TFT) arranges tiny transistors and capacitors in a matrix on the glass of the display. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. The other type of LCD technology is passive matrix. This type of LCD display uses a grid of conductive metal to charge each pixel. Although they are less expensive to produce, passive matrix monitors are rarely used today due to the technology's slow response time and imprecise voltage control compared to active matrix technology. Now that you have an understanding of how LCD technology works, let's look at some specific features unique to LCD monitors. LCD Features and Attributes To evaluate the specifications of LCD monitors, here are a few more things you need to know. Native Resolution Unlike CRT monitors, LCD monitors display information well at only the resolution they are designed for, which is known as the native resolution. Digital displays address each individual pixel using a fixed matrix of horizontal and vertical dots. If you change the resolution settings, the LCD scales the image and the quality suffers. Native resolutions are typically: 17 inch = 1024x768 19 inch = 1280x1024 20 inch = 1600x1200 Viewing Angle When you look at an LCD monitor from an angle, the image can look dimmer or even disappear. Colors can also be misrepresented. To compensate for this problem, LCD monitor makers have designed wider viewing angles. (Do not confuse this with a widescreen display, which means the display is physically wider.) Manufacturers give a measure of viewing angle in degrees (a greater number of degrees is better). In general, look for between 120 and 170 degrees. Because manufacturers measure viewing angles differently, the best way to evaluate it is to test the display yourself. Check the angle from the top and bottom as well as the sides, bearing in mind how you will typically use the display. Brightness or Luminance This is a measurement of the amount of light the LCD monitor produces. It is given in nits or one candelas per square meter (cd/m2). One nit is equal to on cd/m2. Typical brightness ratings range from 250 to 350 cd/m2 for monitors that perform general-purpose tasks. For displaying movies, a brighter luminance rating such as 500 cd/m2 is desirable. Contrast Ratio The contrast ratio rates the degree of difference of an LCD monitor's ability to produce bright whites and the dark blacks. The figure is usually expressed as a ratio, for example, 500:1. Typically, contrast ratios range from 450:1 to 600:1, and they can be rated as high as 1000:1. Ratios more than 600:1, however, provide little improvement over lower ratios. Response Rate The response rate indicates how fast the monitor's pixels can change colors. Faster is better because it reduces the ghosting effect when an image moves, leaving a faint trial in such applications as videos or games. Adjustability Unlike CRT monitors, LCD monitors have much more flexibility for positioning the screen the way you want it. LCD monitors can swivel, tilt up and down, and even rotate from landscape (with the horizontal plane longer than the vertical plane) to portrait mode (with the vertical plane longer than the horizontal plane). In addition, because they are lightweight and thin, most LCD monitors have built-in brackets for wall or arm mounting. Besides the basic features, some LCD monitors have other conveniences such as integrated speakers, built-in Universal Serial Bus (USB) ports and anti-theft locks. LCD Terms Bezel - This is the metal or plastic frame surrounding the display screen. On LCD displays, the bezel is typically very narrow. Contrast ratio - The difference in light intensity between white and black on an LCD display is called contrast ratio. The higher the contrast ratio, the easier it is to see details. Ghosting - An effect of slower response times that cause blurring of images on an LCD monitor, it's also known as latency. The effect is caused by voltage temporarily leaking from energized elements to neighboring, non-energized elements on the display. Luminance - Also known as brightness, it is the level of light emitted by an LCD display. Luminance is measured in nits or candelas per square meter (cd/m2). One nit is equal to one cd/m2. Native resolution - This actual measurement of an LCD display, in pixels, is given in horizontal by vertical order. Response time - The speed at which the monitor's pixels can change colors is called response time. It is measured in milliseconds (ms). Stuck pixels - A pixel that is stuck either 'on' or 'off', meaning that it is always illuminated, unlit, or stuck on one color regardless of the image the LCD monitor displays can also be called a dead pixel. VESA mount - With this, you can mount a monitor on a desk or wall. It meets recommendations of the Video Electronics Standards Association (VESA). Viewing angle - It's the degree of angle at which you can view the screen from the sides (horizontal angle) and top/bottom (vertical angle) and continue to see clearly defined images and accurate colors. CRT Monitors A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image. The illustration below shows how this works inside a CRT. The terms anode and cathode are used in electronics as synonyms for positive and negative terminals. For example, you could refer to the positive terminal of a battery as the anode and the negative terminal as the cathode. Display History 101 Displays have come a long way since the blinking green monitors in text-based computer systems of the 1970s. Just look at the advances made by IBM over the course of a decade: In 1981, IBM introduced the Color Graphics Adapter (CGA), which was capable of rendering four colors, and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically. IBM introduced the Enhanced Graphics Adapter (EGA) display in 1984. EGA allowed up to 16 different colors and increased the resolution to 640x350 pixels, improving the appearance of the display and making it easier to read text. In 1987, IBM introduced the Video Graphics Array (VGA) display system. The VGA standard has a resolution of 640x480 pixels and some VGA monitors are still in use. IBM introduced the Extended Graphics Array (XGA) display in 1990, offering 800x600 pixel resolution in true color (16.8 million colors) and 1,024x768 resolution in 65,536 colors. In a cathode ray tube, the "cathode" is a heated filament. The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons generated by an electron gun that naturally pour off a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. This screen is coated with phosphor, an organic material that glows when struck by the electron beam. There are three ways to filter the electron beam in order to obtain the correct image on the monitor screen: shadow mask, aperture grill and slot mask. These technologies also impact the sharpness of the monitor's display. Let's take a closer look at these now. CRT Features and Attributes To evaluate the specifications of CRT monitors, here are a few more things you need to know: Shadow-mask A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display. The unwanted beams are blocked or "shadowed." Aperture-grill Monitors based on the Trinitron technology, which was pioneered by Sony, use an aperture-grill instead of a shadow-mask type of tube. The aperture grill consists of tiny vertical wires. Electron beams pass through the aperture grill to illuminate the phosphor on the faceplate. Most aperture- grill monitors have a flat faceplate and tend to represent a less distorted image over the entire surface of the display than the curved faceplate of a shadow-mask CRT. However, aperture-grill displays are normally more expensive. Slot-mask A less-common type of CRT display, a slot-mask tube uses a combination of the shadow-mask and aperture-grill technologies. Rather than the round perforations found in shadow-mask CRT displays, a slot-mask display uses vertically aligned slots. The design creates more brightness through increased electron transmissions combined with the arrangement of the phosphor dots. Dot pitch Dot pitch is an indicator of the sharpness of the displayed image. It is measured in millimeters (mm), and a smaller number means a sharper image. How you measure the dot pitch depends on the technology used: In a shadow-mask CRT monitor, you measure dot pitch as the diagonal distance between two like- colored phosphors. Some manufacturers may also cite a horizontal dot pitch, which is the distance between two-like colored phosphors horizontally. The dot pitch of an aperture-grill monitor is measured by the horizontal distance between two like- colored phosphors. It is also sometimes are called stripe pitch. The smaller and closer the dots are to one another, the more realistic and detailed the picture appears. When the dots are farther apart, they become noticeable and make the image look grainier. Unfortunately, manufacturers are not always upfront about dot pitch measurements, and you cannot necessarily compare shadow-mask and aperture-grill CRT types, due to the difference in horizontal and vertical measurements. The dot pitch translates directly to the resolution on the screen. If you were to put a ruler up to the glass and measure an inch, you would see a certain number of dots, depending on the dot pitch. Here is a table that shows the number of dots per square centimeter and per square inch in each of these common dot pitches: Dot Pitch Approx. number of pixels/cm2 Approx. number of pixels/in2 .25 mm 1,600 10,000 .26 mm 1,444 9,025 .27 mm 1,369 8,556 .28 mm 1,225 7,656 .31 mm 1,024 6,400 .51 mm 361 2,256 1 mm 100 625 Refresh Rate In monitors based on CRT technology, the refresh rate is the number of times that the image on the display is drawn each second. If your CRT monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the pixels from top to bottom 72 times a second. Refresh rates are very important because they control flicker, and you want the refresh rate as high as possible. Too few cycles per second and you will notice a flickering, which can lead to headaches and eye strain. Because your monitor's refresh rate depends on the number of rows it has to scan, it limits the maximum possible resolution. Most monitors support multiple refresh rates. Keep in mind that there is a tradeoff between flicker and resolution, and then pick what works best for you. This is especially important with larger monitors where flicker is more noticeable. Recommendations for refresh rate and resolution include 1280x1024 at 85 Hertz or 1600x1200 at 75 Hertz. Multiple Resolutions Because a CRT uses electron beams to create images on a phosphor screen, it supports the resolution that matches its physical dot (pixel) size as well as several lesser resolutions. For example, a display with a physical grid of 1280 rows by 1024 columns can obviously support a maximum resolution of 1280x1024 pixels. It also supports lower resolutions such as 1024x768, 800x600, and 640x480. As noted previously, an LCD monitor works well only at its native resolution. LCDs vs. CRTs If you are looking for a new display, you should consider the differences between CRT and LCD monitors. Choose the type of monitor that best serves your specific needs, the typical applications you use, and your budget. Advantages of LCD Monitors Require less power - Power consumption varies greatly with different technologies. CRT displays are somewhat power-hungry, at about 100 watts for a typical 19-inch display. The average is about 45 watts for a 19-inch LCD display. LCDs also produce less heat. Smaller and weigh less - An LCD monitor is significantly thinner and lighter than a CRT monitor, typically weighing less than half as much. In addition, you can mount an LCD on an arm or a wall, which also takes up less desktop space. More adjustable - LCD displays are much more adjustable than CRT displays. With LCDs, you can adjust the tilt, height, swivel, and orientation from horizontal to vertical mode. As noted previously, you can also mount them on the wall or on an arm. Less eye strain - Because LCD displays turn each pixel off individually, they do not produce a flicker like CRT displays do. In addition, LCD displays do a better job of displaying text compared with CRT displays. Advantages of CRT Monitors Less expensive - Although LCD monitor prices have decreased, comparable CRT displays still cost less. Better color representation - CRT displays have historically represented colors and different gradations of color more accurately than LCD displays. However, LCD displays are gaining ground in this area, especially with higher-end models that include color-calibration technology. More responsive - Historically, CRT monitors have had fewer problems with ghosting and blurring because they redrew the screen image faster than LCD monitors. Again, LCD manufacturers are improving on this with displays that have faster response times than they did in the past. Multiple resolutions - If you need to change your display's resolution for different applications, you are better off with a CRT monitor because LCD monitors don't handle multiple resolutions as well. More rugged - Although they are bigger and heavier than LCD displays, CRT displays are also less fragile and harder to damage. So now that you know about LCD and CRT monitors, let's talk about how you can use two monitors at once. They say, "Two heads are better than one." Maybe the same is true of monitors! Dual Monitors One way to expand your computer's display is to add a second monitor. Using dual monitors can make you more productive and add a lot to your computing experience. With two monitors, you can: View large spreadsheets Make changes to a web page's code on one monitor and view the results on the second Open two different applications, such as a Word document on one monitor and your web browser on the second Besides two displays and two sets of the appropriate video cables, the only other thing you need is a video adapter with two display connections. The connections can be analog or digital; they need only to match the type of connections on the monitors. It does not matter what type of monitor you use; two LCDs, two CRTs, or one of each works fine as long as the video adapter has compatible connections. If you don't have a video adapter with two connections, you can purchase one and replace your current adapter. This generally works better than simply installing another video card with a single connection. Combination cards also come with more features, such as a TV-out port. In addition to verifying your hardware, you should also double-check your computer's operating system to be sure it supports the use of dual monitors. For example, Windows 98 SE, Me, 2000, and XP support multiple monitors. If you really want to increase your screen real estate, especially for applications such as financial trading or 3-D design, you can even implement three or more monitors. Other Technologies Touch-screen Monitors Displays with touch-screen technology let you input information or navigate applications by touching the surface of the display. The technology can be implemented through a variety of methods, including infrared sensors, pressure-sensitive resistors or electronic capacitors. Wireless Monitors Similar in looks to a tablet PC, wireless monitors use technology such as 802.11b/g to connect to your computer without a cable. Most include buttons and controls for mousing and web surfing, and some also include keyboards. The displays are battery-powered and relatively lightweight. Most also include touch-screen capabilities. Television and HDTV Integration Some displays have built-in television tuners that you can use for viewing cable TV on your computer. You can also find displays that accept S-video input directly from a video device. Additional features include picture-in-picture or picture-on-picture capability, a remote control and support for high-definition television (HDTV). VESA Brings Standardization The Video Electronics Standards Association (VESA) is an organization that supports and sets industry-wide interface standards for the PC, workstation and consumer electronics industries. VESA promotes and develops timely, relevant, open standards for the display and display interface industry, ensuring interoperability and encouraging innovation and market growth. In August of 1992, VESA passed the VESA Local Bus (VL-Bus) Standard 1.0. This standard had a significant impact on the industry, because it was the first local bus standard to be developed, which provided a uniform hardware interface for local bus peripherals. The creation of this standard ensured compatibility among a wide variety of graphics boards, monitors, and systems software. Today, VESA is a worldwide organization that promotes and develops open display and display interface standards for interoperability. VESA is a formative influence in the PC industry and a contributor to the enhancement of flat panel display, monitor, graphics, software and systems technologies including home networking and PC theater. Monitor Trends DisplayPort Standard The Video Electronics Standards Association (VESA) is working on a new digital display interface for LCD, plasma, CRT and projection displays. The new technology, which is called DisplayPort, supports protected digital outputs for high definition and other content along with improved display performance. According to VESA, the DisplayPort standard will provide a high-quality digital interface for video and audio content with optional secure content protection. The goal is to enable support for a wide range of source and display devices, while combining technologies. For example, the audio and video signals will be available over the same cable -- a smaller video connector will allow for smaller devices such as notebook computers, and the standard will enable streaming high definition (HD) video and audio content. Organic Light-Emitting Diode Organic Light-Emitting Diodes (OLEDs) are thin-film LED (Light-Emitting Diode) displays that don't require a backlight to function. The material emits light when stimulated by an electrical current, which is known as electroluminescence. OLEDs consist of red, green and blue elements, which combine to create the desired colors. Advantages of OLEDs include lower power requirements, a less-expensive manufacturing process, improvements in contrast and color, and the ability to bend. Surface-Conduction Electron Emitter Displays A Surface-Conduction Electron Emitter Display (SED) is a new technology developed jointly by Canon and Toshiba. Similar to a CRT, an SED display utilizes electrons and a phosphor-coated screen to create images. The difference is that instead of a deep tube with an electron gun, an SED uses tiny electron emitters and a flat-panel display. For more information on computer monitors and related topics, check out the links on the next page. DATAPORTER http://computer.howstuffworks.com/question11.htm If you have a printer connected to your computer, there is a good chance that it uses the parallel port. While USB is becoming increasingly popular, the parallel port is still a commonly used interface for printers. Parallel ports can be used to connect a host of popular computer peripherals: Printers Scanners CD burners External hard drives Iomega Zip removable drives Network adapters Tape backup drives In this article, you will learn why it is called the parallel port, what it does and exactly how it operates. Parallel Port Basics Parallel ports were originally developed by IBM as a way to connect a printer to your PC. When IBM was in the process of designing the PC, the company wanted the computer to work with printers offered by Centronics, a top printer manufacturer at the time. IBM decided not to use the same port interface on the computer that Centronics used on the printer. Instead, IBM engineers coupled a 25-pin connector, DB-25, with a 36-pin Centronics connector to create a special cable to connect the printer to the computer. Other printer manufacturers ended up adopting the Centronics interface, making this strange hybrid cable an unlikely de facto standard. When a PC sends data to a printer or other device using a parallel port, it sends 8 bits of data (1 byte) at a time. These 8 bits are transmitted parallel to each other, as opposed to the same eight bits being transmitted serially (all in a single row) through a serial port. The standard parallel port is capable of sending 50 to 100 kilobytes of data per second. Let's take a closer look at what each pin does when used with a printer: Pin 1 carries the strobe signal. It maintains a level of between 2.8 and 5 volts, but drops below 0.5 volts whenever the computer sends a byte of data. This drop in voltage tells the printer that data is being sent. Pins 2 through 9 are used to carry data. To indicate that a bit has a value of 1, a charge of 5 volts is sent through the correct pin. No charge on a pin indicates a value of 0. This is a simple but highly effective way to transmit digital information over an analog cable in real-time. Pin 10 sends the acknowledge signal from the printer to the computer. Like Pin 1, it maintains a charge and drops the voltage below 0.5 volts to let the computer know that the data was received. If the printer is busy, it will charge Pin 11. Then, it will drop the voltage below 0.5 volts to let the computer know it is ready to receive more data. The printer lets the computer know if it is out of paper by sending a charge on Pin 12. As long as the computer is receiving a charge on Pin 13, it knows that the device is online. The computer sends an auto feed signal to the printer through Pin 14 using a 5-volt charge. If the printer has any problems, it drops the voltage to less than 0.5 volts on Pin 15 to let the computer know that there is an error. Whenever a new print job is ready, the computer drops the charge on Pin 16 to initialize the printer. Pin 17 is used by the computer to remotely take the printer offline. This is accomplished by sending a charge to the printer and maintaining it as long as you want the printer offline. Pins 18-25 are grounds and are used as a reference signal for the low (below 0.5 volts) charge. Notice how the first 25 pins on the Centronics end match up with the pins of the first connector. With each byte the parallel port sends out, a handshaking signal is also sent so that the printer can latch the byte. SPP/EPP/ECP The original specification for parallel ports was unidirectional, meaning that data only traveled in one direction for each pin. With the introduction of the PS/2 in 1987, IBM offered a new bidirectional parallel port design. This mode is commonly known as Standard Parallel Port (SPP) and has completely replaced the original design. Bidirectional communication allows each device to receive data as well as transmit it. Many devices use the eight pins (2 through 9) originally designated for data. Using the same eight pins limits communication to half-duplex, meaning that information can only travel in one direction at a time. But pins 18 through 25, originally just used as grounds, can be used as data pins also. This allows for full-duplex (both directions at the same time) communication. Enhanced Parallel Port (EPP) was created by Intel, Xircom and Zenith in 1991. EPP allows for much more data, 500 kilobytes to 2 megabytes, to be transferred each second. It was targeted specifically for non-printer devices that would attach to the parallel port, particularly storage devices that needed the highest possible transfer rate. Close on the heels of the introduction of EPP, Microsoft and Hewlett Packard jointly announced a specification called Extended Capabilities Port (ECP) in 1992. While EPP was geared toward other devices, ECP was designed to provide improved speed and functionality for printers. In 1994, the IEEE 1284 standard was released. It included the two specifications for parallel port devices, EPP and ECP. In order for them to work, both the operating system and the device must support the required specification. This is seldom a problem today since most computers support SPP, ECP and EPP and will detect which mode needs to be used, depending on the attached device. If you need to manually select a mode, you can do so through the BIOS on most computers. For more information on parallel ports and related topics, check out the links on the next page. Just about any computer that you buy today comes with one or more Universal Serial Bus connectors on the back. These USB connectors let you attach everything from mice to printers to your computer quickly and easily. The operating system supports USB as well, so the installation of the device drivers is quick and easy, too. Compared to other ways of connecting devices to your computer (including parallel ports, serial ports and special cards that you install inside the computer's case), USB devices are incredibly simple! In this article, we will look at USB ports from both a user and a technical standpoint. You will learn why the USB system is so flexible and how it is able to support so many devices so easily -- it's truly an amazing system! Considered to be one of the most basic external connections to a computer, the serial port has been an integral part of most computers for more than 20 years. Although many of the newer systems have done away with the serial port completely in favor of USB connections, most modems still use the serial port, as do some printers, PDAs and digital cameras. Few computers have more than two serial ports. Two serial ports on the back of a PC Essentially, serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer. In this edition of How Stuff Works, you will learn about the difference between a parallel port and a serial port, what each pin does and what flow control is. UART Needed All computer operating systems in use today support serial ports, because serial ports have been around for decades. Parallel ports are a more recent invention and are much faster than serial ports. USB ports are only a few years old, and will likely replace both serial and parallel ports completely over the next several years. The name "serial" comes from the fact that a serial port "serializes" data. That is, it takes a byte of data and transmits the 8 bits in the byte one at a time. The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8). The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Before each byte of data, a serial port sends a start bit, which is a single bit with a value of 0. After each byte of data, it sends a stop bit to signal that the byte is complete. It may also send a parity bit. Serial ports, also called communication (COM) ports, are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it. Serial devices use different pins to receive and transmit data -- using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time. Using different pins allows for full-duplex communication, in which information can travel in both directions at once. Serial ports rely on a special controller chip, the Universal Asynchronous Receiver/Transmitter (UART), to function properly. The UART chip takes the parallel output of the computer's system bus and transforms it into serial form for transmission through the serial port. In order to function faster, most UART chips have a built-in buffer of anywhere from 16 to 64 kilobytes. This buffer allows the chip to cache data coming in from the system bus while it is processing data going out to the serial port. While most standard serial ports have a maximum transfer rate of 115 Kbps (kilobits per second), high speed serial ports, such as Enhanced Serial Port (ESP) and Super Enhanced Serial Port (Super ESP), can reach data transfer rates of 460 Kbps. The Serial Connection The external connector for a serial port can be either 9 pins or 25 pins. Originally, the primary use of a serial port was to connect a modem to your computer. The pin assignments reflect that. Let's take a closer look at what happens at each pin when a modem is connected. 9-pin connector: Carrier Detect - Determines if the modem is connected to a working phone line. Receive Data - Computer receives information sent from the modem. Transmit Data - Computer sends information to the modem. Data Terminal Ready - Computer tells the modem that it is ready to talk. Signal Ground - Pin is grounded. Data Set Ready - Modem tells the computer that it is ready to talk. Request To Send - Computer asks the modem if it can send information. Clear To Send - Modem tells the computer that it can send information. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected. 25-pin connector: Not Used Transmit Data - Computer sends information to the modem. Receive Data - Computer receives information sent from the modem. Request To Send - Computer asks the modem if it can send information. Clear To Send - Modem tells the computer that it can send information. Data Set Ready - Modem tells the computer that it is ready to talk. Signal Ground - Pin is grounded. Received Line Signal Detector - Determines if the modem is connected to a working phone line. Not Used: Transmit Current Loop Return (+) Not Used Not Used: Transmit Current Loop Data (-) Not Used Not Used Not Used Not Used Not Used Not Used Not Used: Receive Current Loop Data (+) Not Used Data Terminal Ready - Computer tells the modem that it is ready to talk. Not Used Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected. Not Used Not Used Not Used: Receive Current Loop Return (-) Voltage sent over the pins can be in one of two states, On or Off. On (binary value "1") means that the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is transmitting a signal between +3 and +25 volts... Going With The Flow An important aspect of serial communications is the concept of flow control. This is the ability of one device to tell another device to stop sending data for a while. The commands Request to Send (RTS), Clear To Send (CTS), Data Terminal Ready (DTR) and Data Set Ready (DSR) are used to enable flow control. Let's look at an example of how flow control works: You have a modem that communicates at 56 Kbps. The serial connection between your computer and your modem transmits at 115 Kbps, which is over twice as fast. This means that the modem is getting more data coming from the computer than it can transmit over the phone line. Even if the modem has a 128K buffer to store data in, it will still quickly run out of buffer space and be unable to function properly with all that data streaming in. With flow control, the modem can stop the flow of data from the computer before it overruns the modem's buffer. The computer is constantly sending a signal on the Request to Send pin, and checking for a signal on the Clear to Send pin. If there is no Clear to Send response, the computer stops sending data, waiting for the Clear to Send before it resumes. This allows the modem to keep the flow of data running smoothly. PCMIA http://www.pcmcia.org/about.htm What is PCMCIA? History PCMCIA (Personal Computer Memory Card International Association) is an international standards body and trade association with over 200 member companies that was founded in 1989 to establish standards for Integrated Circuit cards and to promote interchangeability among mobile computers where ruggedness, low power, and small size were critical. As the needs of mobile computer users has changed, so has the PC Card Standard. By 1991, PCMCIA had defined an I/O interface for the same 68 pin connector initially used for memory cards. At the same time, the Socket Services Specification was added and was soon followed by the Card Services Specifcation as developers realized that common software would be needed to enhance compatibility. In more recent years, PCMCIA has realized the need for higher speed applications such as multimedia and high-speed networking. From this realization came the CardBus and Zoomed Video Specifications which allow blazing speed in such applications as MPEG video and 100 Mbit Ethernet. Along with these speed enhancements, PCMCIA has continued to add to its specification to enhance compatibility and allow for such other mobile-oriented concerns as 3.3V operation and Power Management. Today, PCMCIA promotes the interoperability of PC Cards not only in mobile computers, but in such diverse products as digital cameras, cable TV, set-top boxes, and automobiles. As the variety of products that need modular peripheral expansion has grown, so has the diversity of the capabilities of modular peripherals. As such, PCMCIA has recently changed its mission statement: "To develop standards for modular peripherals and promote their worldwide adoption." PCMCIA's new mission is exemplified by its work with standards for small form factor cards. PCMCIA has added the Small PC Card form factor specifications to the PC Card Standard and now publishes and maintains the Miniature Card Standard. Also, PCMCIA will be publishing the SmartMedia Card Standard which already provides memory solutions in one of the smallest modular peripheral form factors today. All of these cards enable hand-held devices such as digital cameras to use a very small, rugged form of memory while PC Cards will allow the data to be easily transferred to your personal computer through inexpensive adapters. As computing needs become faster and smaller, PCMCIA continues to set the standard. -------------------------------------------------------------------------------- Activities & Publications PC Card Standard PCMCIA publishes the PC Card Standard which contains all of the physical, electrical and software specifications for the PC Card technology. The Standard is constantly being improved by PCMCIA’s technical committee, which meets six times a year. The PC Card Standard can be ordered through this site. PC Card Resource Directory The definitive resource for locating PC Card products and services. With detailed listings from PCMCIA's members, the directory is the industry's most comprehensive source for PC Card product information. Home Page for PC Card Technology PCMCIA hosts a World Wide Web site that includes information about the association, the interactive PC Card Resource Directory, and a complete directory of PCMCIA's members. Trade Shows PCMCIA promotes PC Card technology at trade shows throughout the year. This year, PCMCIA will exhibit at the WinHEC and IDF conferences. Contact the PCMCIA office at firstname.lastname@example.org for more information.