Compaq 108164-003 - ProLiant - 800 Technical Manual page 10

I/O Performance Tuning of Compaq Servers
SCSI Bus Interface
Since SCSI was introduced, several specifications have been released and many new extensions
have been defined. With each subsequent release of the SCSI specification; expandability,
performance, flexibility, and compatibility have increased or improved. Currently, there are three
SCSI specifications.
SCSI-1
The original SCSI standard, approved by ANSI in 1986, defined the first SCSI bus in terms of
cabling length, signaling characteristics, commands, and transfer modes. The default (Regular)
speed for SCSI was 5 MB/s. It had an 8-bit (Narrow) parallel bus that transferred a single byte of
data with each bus cycle. "Regular" and "Narrow" conventions are no longer mentioned in the
SCSI protocol names
SCSI-2
The second version of the SCSI standard, SCSI-2, was approved in 1990. SCSI-2 was an
extensive enhancement that defined support for many advanced features, including:
Fast SCSI: A high-speed transfer protocol that doubles the speed of the bus to 10 MHz.
With an 8-bit data pathway, the transfer rate is 10 MB/s.
Wide SCSI: Widens the original 8-bit SCSI bus to 16 bits to permit more data
throughput at a given signaling speed. The combination of Fast and Wide (Fast-Wide
SCSI-2) offers data transfer rates up to 20 MB/s
More Devices per Bus: Wide SCSI busses support 16 devices (15 drives, plus
controller) as opposed to eight with regular (Narrow) SCSI
Better Cables and Connectors: SCSI-2 defined a new high-density 68-pin "B" cable
and connectors
Active Termination: Provided more reliable termination of the bus.
In addition to these features, SCSI-2 maintained backward compatibility with all SCSI devices.
SCSI-3
SCSI-3 is a group of documents that define the implementation of SCSI protocols on different
physical layers (SCSI-3 Parallel Interface, High Performance Serial Bus, Fibre Channel, and
Serial Storage Architecture). Each physical layer has different performance characteristics and
uses different hardware. Other documents in the SCSI-3 standard are still being developed.
Currently, the SCSI-3 standard includes SCSI-2's performance and functionality enhancements
plus
:
Ultra SCSI: Doubles the bus speed to 20 MHz and the transfer rate to 20 MB/s with an
8-bit data pathway
Wide-Ultra SCSI-3: Doubles the Ultra SCSI transfer rate to 40 MB/s using a 16-bit data
pathway
Improved Cabling: A new 68-pin "P" cable replaces the "B" cable for use with Wide
SCSI.
Compaq has extensively tested and integrated the Wide-Ultra SCSI-3 technology in Compaq
servers and storage options because it allows the highest available performance in a SCSI host
ECG044.0399
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