IBM HANDBOOK 260 - Manual (Page 120)

102 RS/6000 43P 7043 Models 150 and 260 Handbook
These three serial SCSI interfaces have two major differences over their
parallel sibling:
• The cable has only six wires
• The connectors are simplified into six pins
These new physical properties translate into lower costs for users and
manufacturers.
You should take note that SCSI-III Parallel Interface (SPI) is backwardly
compatible with SCSI-I and SCSI-II. It is ideal for those users that have
sizable investments in SCSI-II equipment because SPI integrates SCSI-II and
SCSI-III devices on the same chain. The use of SCSI-III provides much more
function due to the improvements in the command set and data transfer rates.
The migration from SCSI-II to SPI is relatively simple because all the
connectors, cables and terminators essentially remain the same.
The SCSI-III Parallel Interface specification uses the terms Fast-20 Narrow and
Fast-20 Wide . Fast-20 Narrow is an 8-bit bus with a maximum data transfer rate
of 20 MB/s. Fast-20 Wide is a 16-bit bus with a maximum data transfer rate of
40 MB/s. The
20 in Fast-20 refers to the clock speed of bus: 20 MHz, which is
double the speed of SCSI-II Fast.
The term Ultra SCSI is another name for Fast-20 Wide. Ultra SCSI is not
equivalent to SCSI-III, but is a subset of the SCSI-III Parallel Interface (SPI).
4.2.4 SCSI Terminology
The terminology of SCSI products can sometimes be confusing. Here are
some definitions to help your understanding.
4.2.4.1 General Terminology
The following is a list of general terminology for SCSI.
Fast Fast refers to the doubling of the data transfer rate from
the SCSI 5 MB/s to 10 MB/s by doubling the clock rate.
SCSI (that is, the original SCSI specification, or SCSI-I) is
5 MB/s, which is produced by a clock speed of 5 MHz
sending data down eight wires. SCSI-II Fast achieves 10
MB/s by doubling the clock speed to 10 MHz.
Wide Wide is used in reference to the width of the SCSI parallel
bus between the adapter and the device. Wide means
wider than the original 8-bit path defined in SCSI-I, usually
16-bit. 32-bit transmission is possible within the
specification but there are no "Wide-32" devices on the