PC Hardware in a Nutshell, 3rd Edition

SCSI Standards

SCSI is confusing because of the proliferation of terms, many of which refer to similar things in different ways or to different things in similar ways. There are actually three SCSI standards, each of which refers not to any particular implementation, but to the document that defines that level.

SCSI-1

The SCSI standard was adopted in 1986 and is now obsolete. Originally called simply SCSI, but now officially SCSI-1, this standard defines a high-level method of communicating between devices, an Initiator (normally a computer), and a Target (normally a disk drive or other peripheral). SCSI-1 permits data to be transferred in asynchronous mode (unclocked mode) or synchronous mode (clocked mode), although commands and messages are always transferred in asynchronous mode. SCSI-1 uses the low-density 50-pin connector for both internal and external connections. The external low-density 50-pin connector is also referred to as the Centronics SCSI connector. SCSI-1 is a single comprehensive document that defines all physical and protocol layers, and is published as ANSI X3.131-1986.

SCSI-2

SCSI-2 was adopted in 1994, and many current SCSI devices are SCSI-2 compliant. SCSI-2 updated the SCSI-1 standard to include faster data rates and to more tightly define message and command structures for improved compatibility between SCSI devices. SCSI-2 devices use various connectors, depending on the width and speed of the implementation. SCSI-2 is a single comprehensive document that ...