Rules 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 two ends.
- 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.
- 8-bit (Narrow) 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.
- 16-bit (Wide) 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.
- High Voltage Differential (HVD) (also known as
"Differential") SCSI drives and controllers should not be
mixed with other SCSI hardware. A Differential bus requires
Differential terminators.
- Low Voltage Differential (LVD) SCSI devices
require LVD cables and terminators. LVD SCSI hardware should not be
confused with "Differential" hardware.
- Most SCSI devices are Single-Ended (SE), which
means they use "normal" electrical signals (not HVD or LVD as
described previously).
