SCSI by gegeshandong


           Small Computers System Interface
         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
 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
     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

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-

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)

Required to eliminate interference

Two main types; Passive, Active

Passive used for short systems running at the usual bus

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)

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
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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