Originally From: "SmithDan" [firstname.lastname@example.org]
SCSI Facetiously Answered Questions
Written by Daniel P. B. Smith; inspired by Bill McGee, John Morton,
and Jim Guerrera.
- 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
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?
- Straight and narrow SCSI. Losing popularity because it is
unreliable in typical SCSI setups, in which the cables have bends
- High, wide and handsome SCSI. Expensive variety typically used in
high-end Digital and Sun systems.
- 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
- Free and easy SCSI. Built in to Apple Computer, Inc. Macintoshes
at no additional charge.
- 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.
- Single-ended SCSI. No longer used, because in practice it was 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 follows 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:
- Shock. The initiator asserts 117 VAC on the high-attention line.
- Denial. The target rejects the request by deserting the cowardly
- Anger. The initiator connects the +5V rail to the Maginot line,
burning its logical unit number into the fusible links of the
- Initiator informs the target that it supports high/wide
- 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.
- Grieving. Signals are highly depressed by initiator. The target
acknowledges it is on a sinking chip by desponding within 150
- 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
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
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