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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standard was originally released in 1990 as X3.131-1990, but it was retracted for further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:
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.