The Small Computer
System Interface or SCSI is an interface technology which
allows you to attach a wide variety of internal and external
devices to a single controller card in your computer. SCSI
hardware originated as the drive interface technology of
choice for high-end PCs and server computers because of its
greater flexibility and faster performance. It is also well
suited to create and support high speed RAID arrays.
A typical SCSI
configuration consists of the following items:
A SCSI host
adapter/controller that plugs into a slot in your computer's
external SCSI devices (such as hard drives, CD-ROM, DVD,
external SCSI cables, SCSI terminators, and SCSI cable
One has to be
careful in their selection of SCSI hardware in order to ensure
proper operation. You should check your SCSI device
specifications and documentation in order to look for special
are three distinct types of SCSI signals:
Single Ended (SE)
Most SCSI devices use "normal" SE signalling, which limits
the maximum length of a SCSI bus to 1.5m (4.9ft). This
includes most 50-pin (Narrow) SCSI devices such as scanners
and Zip drives.
High Voltage Differential (HVD)
(also known as "Differential") A Differential SCSI system
provides reliable signalling in high noise environments over
a long bus length (up to 25m [82ft]). HVD hardware cannot be
mixed with other SCSI signal types. A Differential bus
requires Differential terminators.
Differential (LVD) A typical multimode LVD/SE SCSI
system provides a moderately long bus length (up to 12m
[39ft]) and downward compatibility with SE hardware.
LVD-rated equipment is required for "Ultra" SCSI standards.
of SCSI Connectors
There are various
different connector types for each sort of SCSI variant.
All the SCSI
connectors can be found if you
for Connecting SCSI Devices
SCSI is a "bus"
architecture, which means devices are connected along a line
that has a beginning and an end. This cabling scheme is
commonly called a "daisy chain".
The two extreme
ends of a SCSI bus segment must be properly "terminated". A
terminator is a small device designed to dampen electrical
signals reflected from the ends of a cable. Termination is
disabled for any SCSI device that is positioned between the
The SCSI host
adapter (or controller) may be positioned anywhere along the
bus. It must be terminated if it is positioned at the end of
the bus. Many host adapters provide automatic termination.
SCSI devices require 50-pin (or fewer) connections. Up to 7
different devices may be controlled in a Narrow bus. Narrow
SCSI signals include Low Byte & Control.
SCSI devices require 68-pin connections. Up to 15 different
devices may be controlled in a Wide bus. Wide SCSI signals
include Low Byte & Control, and High Byte.
SCSI is a
downwards-compatible technology. Older SCSI devices can be
installed in a newer (and faster) SCSI bus segment, but
overall system performance may be reduced.
Each device on a
SCSI bus must be assigned a unique identifier (SCSI ID)
which is usually set by jumper caps. The ID is independent
of the device's relative position on the bus. When mixing
Narrow and Wide devices on the same bus, the ID of the host
adapter should be set to 7 (or less) so that the Narrow
devices can recognize it.
50-pin to 68-pin
SCSI cable adapters usually allow Narrow devices to be used
in a Wide bus. Termination of the High Byte signal must be
considered if a Narrow SCSI device is positioned at the end
of a Wide SCSI bus.
68-pin to 50-pin
SCSI cable adapters usually allow Wide devices to be used in
a Narrow bus. The speed of the Wide SCSI device will be
limited to that of the Narrow SCSI bus.
Differential (HVD) (also known as "Differential") SCSI
drives and controllers should not be mixed with other SCSI
hardware. A Differential bus requires Differential
Differential (LVD) SCSI devices require LVD cables and
terminators. LVD SCSI hardware should not be confused with
devices are Single-Ended (SE), which means they use "normal"
electrical signals (not HVD or LVD as described previously).