SCSI

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SCSI

• Small Computers System Interface
• Creation
• Evolution
• Application

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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

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Incarnations of SCSI standards, 1986 to present
(Will be explained later in detail, no need to
copy yet)
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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

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Basic Pros/Cons of SCSI
Pros of SCSI
Cons of SCSI

• Fast transfer speeds, up to 320 megabytes per
  second
• Reliable, durable components
• can connect many devices with a single bus, more
  than just HDs
• SCSI host cards can be put in almost any system
• Full backwards compatibility

• Configuration and setup more picky and specific
  to one computer
• Unlike IDE, few BIOS support the standard
• Overwhelming number of variations in the
  standard, hardware, and connectors
• No common software interfaces and protocol

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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

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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 theres something out there
  now with !32-bit! datapaths, meaning simply,
  0-31)

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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.

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Evolution of SCSI Recap
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SCSI Components Controller

• Also called Host Adapter
• The strongest link in a SCSI chain, gives SCSI
  its 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

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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 IDs in
  most cases, but Plug and Play SCSI does exist
  Assigns fixed IDs first, then fills in later
  devices.

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SCSI Components Cables
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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)

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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

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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)

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SCSI NOW

• Used in servers and workstations
• Offers flexibility, performance, and backwards
  compatibility
• Not a good choice for the average IMing
  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.

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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 its name
• SCSI is a quickly evolving animal über wide SCSI
  and yet faster transfer speeds.

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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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