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8.1.3 SCSI-I vs SCSI-II vs SCSI-III




Description

This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.

8.1.3 SCSI-I vs SCSI-II vs SCSI-III

    SCSI (now commonly known as SCSI-I) was the original 1986 standard,
X3.131-1986.  It specified the electrical level and some of the
mid-layer issues involving messages and packet structure, but (I
believe, my memory's bad) didn't formalize the Common Command Set
(CCS), that was done independently. It supported a maximum burst rate
of 5 MB/sec. on an 8-bit bus.

ADDITIONAL INFORMATION

Consult the SCSI standards documents, and the manuals for the device you
are working with for more information.  The "SCSI 1" specification
document is called SCSI Specification, ANSI X3T9.2/86-109. Also of
interest is the Common Command Set specification document SCSI CCS
Specification, ANSI X3T9.2/85-3

SCSI-II received final approval in early 1994, but has been a de facto
standard for several years. The CCS was standardized for a variety of
different types of peripherals. The max allowable transfer rate was
raised to 10 MT/s (see below). A 16-bit bus (Wide SCSI) and 32-bit bus
(double-wide SCSI) are specified (see below).

SCSI-III is the latest effort, and involves more cleanly separating
the functionality into layers; the command layer is defined
independently from the physical layer. In addition to the traditional
parallel cable, there are efforts going on to define physical layers
for Fibre Channel and a more generic Serial SCSI. Thus, there will be
no SCSI-IV; only the individual pieces will be updated as necessary.
    

 

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