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SCSI Small Computers System Interface Creation Evolution Application What is SCSI? Like IDE, an I/O bus standard Bus: the channel or path between the components in a computer. Developed in 1986, continues to evolve today Incarnations of SCSI standards, 1986 to present (Will be explained later in detail, no need to copy yet) The origins of SCSI: Originally developed by Shugart Associates and NCR in 1981 Known as ‘Shugart Associates System Interface” (SASI) SASI designed to be a high-performance, proprietary bus American National Standards Institution (ANSI) ratified an improved version, called SCSI or, ‘small computer system interface’ in 1986 Basic Pros/Cons of SCSI: Pros of SCSI Cons of SCSI Fast transfer speeds, up to 320 Configuration and setup more megabytes per second picky and specific to one computer Reliable, durable components Unlike IDE, few BIOS support can connect many devices with a the standard single bus, more than just HDs Overwhelming number of SCSI host cards can be put in variations in the standard, almost any system hardware, and connectors Full backwards compatibility No common software interfaces and protocol Evolution of SCSI: SCSI-1 : 1986, defines the basics of the first SCSI buses, including cable length, signaling characteristics, commands and transfer modes. 5 MB/sec data transfer, 8-bit wide bus The Original platform that SCSI was built around, remained standard for four years, until 1990 Not incredibly reliable, very confusing command set Still compatible with modern equipment, specifically SCSI-2 cabling, but degrades performance Evolution of SCSI (cont): SCSI 2: 1990 (‘94) Prime advancement is Standardized “Common Command Set (CCS)” of 18 bare-bones commands Introduced the first of the SCSI variants: “Wide” SCSI has 16-bit datapath, doubling device limits from 0-7 to new 0-15 “Fast” SCSI doubles clock speed from 5-10mhz, increasing speed. Can be used alone, or together Introduced Command Queuing- Queues up commands, and decides which ones had priority (*Groans* apparently there’s something out there now with !32-bit! datapaths, meaning simply, 0-31…) Evolution of SCSI (cont): SCSI 3: 1995 become a category of other, related standards SPI: SCSI parallel interface, communications standard between devices. ‘Ultra SCSI’ designates SPI-1, ‘Ultra-2’ SPI-2 and so on Wide/Fast still apply, but fast is double the devices /Base/ bus speed. Evolution of SCSI Recap SCSI Components; Controller Also called Host Adapter The strongest link in a SCSI chain, gives SCSI it’s advantages Expansion card or built in chip, contains SCSI BIOS in ROM or Flash ROM. Software allows it to manage the SCSI Chain Some devices have ‘mini controllers’ that talk to Host adapter. These are called ‘Embedded SCSI devices’ SCSI Components; Devices SCSI supports hard drives, Scanners, CD-ROMS, external drives, and many other components Each device on a chain requires a unique ID. SCSI can support devices numbered 0-7, with wide SCSI supporting numbers 0- 15. ONE ID, TYPICALLY THE HIGHEST, IS RESERVED FOR THE HOST ADAPTER ITSELF Hardware or Software settings determine ID’s in most cases, but Plug and Play SCSI does exist; Assigns fixed ID’s first, then fills in later devices. SCSI Components; Cables SCSI Components; Cables 7 types of cables: DB-25 (SCSI-1) 50-pin internal ribbon (SCSI-1, SCSI-2, SCSI-3) 50-pin Alternative 2 Centronics (SCSI-1) 50-pin Alternative 1 high density (SCSI-2) 68-pin B-cable high density (SCSI-2) 68-pin Alternative 3 (SCSI-3) 80-pin Alternative 4 (SCSI-2, SCSI-3) Termination Required to eliminate interference Two main types; Passive, Active Passive used for short systems running at the usual bus speed Active used for long cables or high-speed busses Also depends on Bus Type SCSI Bus Signaling AKA, Tribes Three types of Signaling SE or Single Ended: Card sends a single pulse, each device taking part of the signal. Degrades rapidly. (10 ft) HVD or High Voltage Differential; Two data lines, each device re-transmits signal down line. (up to about 80 feet) LVD or Low Voltage Differential; Same principle as HVD but smaller re-transmitters, lower voltage. More cost effective, but shorter length of chain (40 feet) SCSI NOW: Used in servers and workstations Offers flexibility, performance, and backwards compatibility Not a good choice for the average IM’ing web-surfing game- playing User. Only the über-users need apply Long life and endurance appeals to businesses. More complex, and less friendly to home users. SCSI in the Future: Will probably remain Commercial, not residential Still a valuable tool in Servers, Workstations Will probably always seem expensive, ‘cause IDE is cheap No matter what we build in the future, there will probably always be something reminiscent of SCSI that bears it’s name SCSI is a quickly evolving animal: über wide SCSI and yet faster transfer speeds. Getting it to WORK: "SCSI is *not* magic. There are *fundamental* *technical* reasons why you have to sacrifice a young goat to your SCSI chain every now and then." -John F. Woods I. Thou shalt terminate both ends (and ONLY the ends) of thine SCSI bus. If thine bus is WIDE, be sure that both the lower half and upper half of the bus are properly terminated. II. Thou shalt use active (or LVD/SE) terminators wherever possible. III. Thou shalt make certain that each SCSI device has a SCSI ID unique unto itself. IV. Thou shalt make certain that terminator power is supplied by at least one device (laptop users pay special heed!) V. Thou shalt not make thine SCSI bus longer than the speed of thy fastest device allows. These being: 6 meters for 5 MHz (SCSI-1 synch) 3 meters for 10 MHz (SCSI-2 Fast) 1.5 meters for 20 MHz (Fast-20/Ultra) 12 meters for 40 MHz (Fast-40/Ultra2) or higher using LVD. VI. Thou shalt not mix the tribes of SCSI! HVD cannot abide SE or LVD and neither can they abide HVD. Be sure you know which tribe each of your devices is from. HVD is now considered to be in exile and should be shunned if sighted in your vicinity. VII. Thou shalt keep thine device drivers up to date (But keep a known working version around too). VIII. Thou shalt make certain that the power supplies for your devices are adequate to supply their needs (even during peak usage). IX. Thou shalt not worship false connectors. A special place in SCSI Hell is reserved for designers using 25 pin connectors on SCSI devices! X. Thou shalt double check that thou hast obeyed all of the above before posting a question to the enlightened in the comp.periphs.scsi newsgroup!
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