SCSI Facetiously Answered Questions
Written by Daniel P. B. Smith; inspired by Bill McGee, John Morton, and
Jim Guerrera.
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. Return
to contents
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." Return to contents
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 in
them.
- 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 configuration.
- 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.
Return to contents
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 line.
- Anger. The initiator connects the +5V rail to
the Maginot line, burning its logical unit number into the fusible links
of the target PROM.
- 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.
- Grieving. Signals are highly depressed by
initiator. The target acknowledges it is on a sinking chip by desponding
within 150 nanoseconds.
- Acceptance. After time for reflection,
target data reaches the terminator, where it is absorbed and
vanishes.
Return to contents
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.
Return to contents
Date created: 9/13/96 Last modified: 9/13/96 Copyright © 1996,
Daniel P. B. Smith Maintained by: Daniel P. B. Smith dpbsmith@world.std.com