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You cannot mix single-ended and differential devices on a single SCSI bus; the result would be catastrophic. (That is to say, you probably will see smoke!) Notice that the cables and connectors are the same, so it's entirely possible to make this mistake. This usually is not a problem, however, because very few differential SCSI implementations exist. Especially with SCSI in the PC environment, single-ended is about all you will ever see. If, however, you to come upon a peripheral that you believe might be differential, there are a few ways to tell. One way is to look for a special symbol on the unit; the industry has adopted different universal symbols for single-ended and differential SCSI. Figure 2-1 shows these symbols.

FIG. 2-1.  Single-ended and differential SCSI universal symbols.

If you do not see such symbols, you can tell whether you have a differential device by using an ohmmeter to check the resistance between pins 21 and 22 on the device connector. On a single-ended system, the pins should be tied together and also tied to the ground. On a differential device, the pins should be open or have significant resistance between them. Again, this generally should not be a problem, because virtually all devices used in the PC environment are single-ended.

SCSI-1 and SCSI-2

The SCSI-2 specification essentially is an improved version of SCSI-1 with some parts of the specification tightened and with several new features and options added. Normally, SCSI-1 and SCSI-2 devices are compatible, but SCSI-1 devices ignore the additional features in SCSI-2.

Some of the changes in SCSI-2 are very minor. For example, SCSI-1 allowed SCSI Bus parity to be optional, whereas parity must be implemented in SCSI-2. Another requirement is that initiator devices, such as host adapters, provide terminator power to the interface; most devices already did so.

SCSI-2 also has several optional features:

  Fast SCSI
  Wide SCSI
  Command queuing
  High-density cable connectors
  Improved Active (Alternative 2) termination

These features are not required; they are optional under the SCSI-2 specification. If you connect a standard SCSI host adapter to a Fast SCSI drive, for example, the interface will work, but only at standard SCSI speeds.


SCSI-3 is a term used to describe a set of standards currently being developed. Simply put, it is the next generation of documents a product conforms to. (See the "New Commands" section later in this chapter.)

Fast and Fast-Wide SCSI

Fast SCSI refers to high-speed synchronous transfer capability. Fast SCSI achieves a 10MBps transfer rate on the standard 8-bit SCSI cabling. When combined with a 16-bit Wide SCSI interface, this configuration results in data-transfer rates of 20MBps (called Fast/Wide).

Fast-20 (Ultra) SCSI

Fast-20 or Ultra SCSI refers to high-speed synchronous transfer capability that is twice as fast as Fast-SCSI. This has been introduced in the Draft (unfinished) SCSI-3 specification and has already been adopted by the marketplace, especially for high-speed hard disks. Ultra SCSI achieves a 20MBps transfer rate on the standard 8-bit SCSI cabling. When combined with a 16-bit Wide SCSI interface, this configuration results in data-transfer rates of 40MBps (called Ultra/Wide).

Fast-40 SCSI

Fast-40 SCSI is a future revision of SCSI-3 (mentioned earlier in the chapter) capable of achieving a 40MBps transfer rate.


Wide SCSI allows for parallel data transfer at a bus width of 16 bits. The wider connection requires a new cable design. The standard 50-conductor 8-bit cable is called the A cable. SCSI-2 originally defined a special 68-conductor B cable that was supposed to be used in conjunction with the A cable for wide transfers, but the industry ignored this specification in favor of a newer 68-conductor P cable that was introduced as part of the SCSI-3 specification. The P cable superseded the A and B cable combination because the P cable can be used alone (without the A cable) for 16-bit Wide SCSI.

A 32-bit Wide SCSI version was originally defined on paper as a part of the SCSI-2 specification, but has not found popularity and probably never will in the PC environment. Theoretically, 32-bit SCSI implementations would require two cables: a 68-conductor P cable and a 68-conductor Q cable.

Fiber Channel SCSI

Fiber Channel SCSI is a specification for a serial interface using a fiber channel physical and protocol characteristic, with SCSI command set. It can achieve 100MBps over either fiber or coaxial cable.


The single-ended SCSI bus depends on very tight termination tolerances to function reliably. Unfortunately, the original 132-ohm passive termination defined in the SCSI-1 document was not designed for use at the higher synchronous speeds now possible. These passive terminators can cause signal reflections to cause errors when transfer rates increase or when more devices are added to the bus. SCSI-2 defines an active (voltage-regulated) terminator that lowers termination impedance to 110 ohms and improves system integrity.

Command Queuing

In SCSI-1, an initiator device, such as a host adapter, was limited to sending one command per device. In SCSI-2, the host adapter can send as many as 256 commands to a single device, which will store and process those commands internally before responding on the SCSI bus. The target device even can resequence the commands to allow for the most efficient execution or performance possible. This feature is especially useful in multitasking environments, such as OS/2 and Windows NT, that can take advantage of this feature.

New Commands

SCSI-2 took the Common Command Set that was being used throughout the industry and made it an official part of the standard. The CCS was designed mainly for disk drives and did not include specific commands designed for other types of devices. In SCSI-2, many of the old commands are reworked, and several new commands have been added. New command sets have been added for CD-ROMs, optical drives, scanners, communications devices, and media changers (jukeboxes).


Even though the SCSI-2 specification has only recently been approved (although it has remained stable for some time), the SCSI-3 specification is already being developed. SCSI-3 will have everything that SCSI-2 has and definitely will add new commands, features, and implementations. For example, SCSI-3 will provide support for up to 32 devices on the bus instead of only eight.

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