Silly SCSI FAQ...

The Loony Bin ( loonies@bloodaxe.demon.co.uk )
Wed, 18 Sep 1996 16:00:04 +0100


Hiya Folks...

Some things are sillier than others...

Wishes & Dreams...

- ANDREA
        xx

************<andrea@bloodaxe.demon.co.uk>************
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  ------- Forwarded foolishness follows -------

              SCSI Facetiously Answered Questions

Written by Daniel P. B. Smith; inspired by Bill McGee, John Morton, and 
Jim Guerrera.

Contents

     * What are the differences between SCSI-1 and SCSI-2?
     * How is "SCSI" pronounced?
     * What are the major varieties of SCSI?
     * What are the stages through which a SCSI transaction proceeds?
     * What can I do to ensure that my SCSI system will work reliably?

     _________________________________________________________________

Q1: What are the differences between SCSI-1 and SCSI-2?

   A: The SCSI standard has evolved over the years. The original
   standard, SCSI-1, was highly successful and popular, but didn't
   actually work. The industry's response to this was SCSI-2. SCSI-2 
   adds support for a number of high-performance modes, defines a common
   command set, and throws in the kitchen sync. In practice, SCSI-2
   doesn't work either. The industry's response to this is SCSI-3, which
   will introduce thicker, sturdier, stronger, heavier, and far more
   expensive cables.

     _________________________________________________________________

Q2: How is "SCSI" pronounced?

   A: At one time, there was a debate as to whether it should be
   pronounced "scuzzy" or "sexy." Now, however, it is universally agreed
   that the correct pronunciation is "scuzzy."

     _________________________________________________________________

Q3: What are the major varieties of SCSI?

    1. Straight and narrow SCSI. Losing popularity because it is
       unreliable in typical SCSI setups, in which the cables have bends
       in them.
    2. High, wide and handsome SCSI. Expensive variety typically used in
       high-end Digital and Sun systems.
    3. Fast and loose SCSI. Often used in PCs. Has a few small 
       variations from the official SCSI specification but works quite 
       well as long as you never install new equipment or change the 
       configuration.
    4. Free and easy SCSI. Built in to Apple Computer, Inc. Macintoshes
       at no additional charge.
    5. Differential SCSI. Incorporates special gear to reduce bit skew 
       by speeding up the signals that are routed on the outside of the
       cable bends and thus must travel a slightly longer distance.
    6. Single-ended SCSI. No longer used, because in practice it was 
       necessary to employ a cable with two ends unless the devices
       being connected were very close together.

     _________________________________________________________________

Q4: What are the stages through which a SCSI transaction proceeds?

   A: SCSI transactions follow a technical protocol that can only be
   expressed accurately in a diagram that has plenty of boxes, lines, 
   and arrows. However, a typical SCSI transaction includes at least the
   following stages:
    1. Shock. The initiator asserts 117 VAC on the high-attention line.
    2. Denial. The target rejects the request by deserting the cowardly
       line.
    3. Anger. The initiator connects the +5V rail to the Maginot line,
       burning its logical unit number into the fusible links of the
       target PROM.
    4. Bargaining:
         + Initiator informs the target that it supports high/wide SCSI.
         + Target replies that it is just a poor, humble device with
           many LUNS to support, and can't possibly afford such an
           expensive protocol. Offers fast/loose SCSI instead.
         + Initiator laughs heartily, but says it would consider
           compromising on free/easy SCSI, if the target would throw in
           five extra bytes.
         + Target says it can do that if the initiator will agree to
           wait an extra 50 nanoseconds for the acknowledgement.
         + Initiator replies with an offer the target can't refuse. They
           handshake.
    5. Grieving. Signals are highly depressed by initiator. The target
       acknowledges it is on a sinking chip by desponding within 150
       nanoseconds.
    6. Acceptance. After time for reflection, target data reaches the
       terminator, where it is absorbed and vanishes.

     _________________________________________________________________

Q5: What can I do to ensure that my SCSI system will work reliably?

   A: Your SCSI system will work reliably as long as you follow a few
   simple rules. First, the cable must be properly terminated. You will
   need at least one terminator. If you are using SCSI-2 you will need a
   Terminator II. On the other hand, if the cable has two ends the other
   end will need a terminator, too. If, on the third hand, the cable is
   single-ended, it should have a terminator on both ends. If it is a
   flat ribbon cable, you can avoid the need for a terminator by giving
   it a half-twist and joining the ends. This is especially appropriate
   when mixing big-endian and little-endian devices on the same SCSI 
   bus.

   Second, be sure to use high-quality cable. To be sure that you get
   high-quality cable, remember to ask your supplier: "Is your cable 
   high quality?" If your supplier says, "Yes, our cable is high 
   quality," you can be assured that their cable is of high quality. On 
   the other hand, you should generally avoid suppliers that say, 
   "Naaah, we charge you for high quality cable but what we actually 
   give you is cheap crap."

   Third, check your cable length. Remember, "measure twice, cut once."
   In calculating length, include the lengths of all stubs. Remember
   these basic rules: no two stubs can be within 41 inches of each other
   and must be at least 115 cm. away from any terminator; total cable
   length should not exceed 9.3 feet; and every pair of communicating
   devices must be within 0.42 meters of each other.

     _________________________________________________________________

Date created: 9/13/96 Last modified: 9/13/96 Copyright ? 1996,
Daniel P. B. Smith Maintained by: Daniel P. B. Smith


PS:  Someone else commented that in cases of bus contention, binding 
arbitration should be enabled.  <grin>  [Jo]