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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Standards ]


In 1985, a year before the SCSI-1 standard was formally approved, work began on the SCSI-2 specification. Important goals of this evolution of the SCSI standard were to improve performance, enhance reliability, and add features to the interface. However, the most important objective was to formalize and properly standardize SCSI commands. After the confusion that arose from the non-standardized implementations of original SCSI, a working paper was created to define a set of standard commands for SCSI hard disks, called the common command set or CCS. This paper eventually formed the basis for the new SCSI-2 standard. SCSI-2 was approved by ANSI in 1994 and released as document X3.131-1994.

Note: The SCSI-2 standard was originally released in 1990 as X3.131-1990, but it was retracted for further changes and didn't actually get formally approved until four years later. You may see reference to the 1990 version of the standard on occasion; there are actually few differences between it and the 1994 version.

SCSI-2 is an extensive enhancement of the very limited original SCSI. The command set used for SCSI devices was standardized and enhanced, and several confusing "options" removed. In addition, the standard defines the following significant new features as additions to the original SCSI-1 specification:

  • Fast SCSI: This higher-speed transfer protocol doubles the speed of the bus to 10 MHz, meaning 10 MB/s transfer rate with 8-bit regular SCSI cabling or even higher when used with Wide SCSI.
  • Wide SCSI: The width of the original SCSI bus was increased to 16 (or even 32) bits. This permits more data throughput at a given signaling speed. Wide SCSI eventually replaced original "narrow" SCSI buses for the fastest drives.
  • More Devices per Bus: On buses that are running with Wide SCSI, 16 devices are supported (as opposed to 8 with regular SCSI).
  • Improved Cables and Connectors: As discussed in detail here, SCSI uses a large number of different cable and connectors. SCSI-2 defined new higher-density connections, extending the basic 50-pin connectors defined in SCSI-1.
  • Active Termination: Termination is an important technical consideration in setting up a SCSI bus. SCSI-2 defined the use of active termination, which provides more reliable termination of the bus.
  • Differential Signaling: To allow longer cable lengths, differential signaling was introduced. (This was later renamed "high-voltage differential" to distinguish it from low voltage differential (LVD) signaling.)
  • Command Queuing: One of SCSI's strengths is its ability to allow multiple outstanding requests between devices on the bus, simultaneously. Command queuing was introduced in SCSI-2.
  • Additional Command Sets: SCSI-2 added new command sets to support the use of more devices such as CD-ROMs, scanners and removable media. The older command set focused more on hard disks.

There were also several other minor changes to the standard, mostly low-level technical changes that I don't really need to get into. It is important to note that one of the major design criteria in the creation of SCSI-2 was backward compatibility with SCSI-1. SCSI-2 devices will in most cases work with older SCSI-1 devices on a bus. This is not always done, however, because the older devices have no ability to support the SCSI-2 enhancements and faster transfer protocols.

Note: SCSI-2 is not the same as Ultra2 SCSI, which is a much newer and higher-performance feature set.

Next: SCSI-3

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