SCSI-2 is officially known as ANSI X3.131-1994. The SCSI-2 specification is essentially an improved version of SCSI-1 with some parts of the specification tightened and 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. Parity is an extra bit that is sent as a verification bit to ensure that the data is not corrupted.

Another requirement is that initiator devices, such as host adapters, provide terminator power to the interface; most devices already did so, though. SCSI-2 also added several optional features:

  • Fast SCSI (10MHz)

  • Wide SCSI (16-bit transfers)

  • Command queuing

  • New commands

  • High-density, 50-pin cable connectors

  • Active (Alternative 2) termination for improved single-ended (SE) transmission

  • High Voltage Differential transmission (incompatible with SE on the same bus) for longer bus lengths

Wide SCSI enables parallel data transfers 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 32-bit wide transfers.

However, because of a lack of industry support and the added expenses involved, 32-bit SCSI was never actually implemented and was finally removed as part of the SCSI-3 specifications. Therefore, two different types of SCSI cables are now available, called the A cable and the P cable.

A cables are any SCSI cables with 50-pin connectors, whereas P cables are any SCSI cables with 68-pin connectors. You need a P cable if you are connecting a Wide SCSI device and want it to work in 16-bit mode. The P cable was not officially included in the standard until SCSI-3.

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).

The high-density connectors enable smaller, more efficient connector and cable designs.

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 can even resequence the commands to enable the most efficient execution or performance possible. This is especially useful in multitasking environments, such as OS/2 and Windows NT, which can take advantage of this feature.

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). 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, which are sometimes built in to SCSI devices, can cause signal reflections to generate 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. Note that LVD SCSI requires special LVD terminators. If you use SE terminators on a bus with LVD devices, they either won't work or, if they are multimode devices, will default to SE operation.

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.