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SCSI-2
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standard was originally released in 1990 as X3.131-1990, but it was retracted for further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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