SCSI FAQ (Facetiously Answered Questions)
Written by Daniel P. B. Smith;
inspired by Bill McGee, John Morton, and Jim Guerrera.

Contents

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 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:
  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.
Source:
Author: Daniel P. B. Smith (dpbsmith@world.std.com), posted 1996 Oct 17 to comp.periphs.scsi and comp.arch.storage
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