Course 2: SCSI School
Rules for connecting SCSI devices
- SCSI is a bus architecture, meaning 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 using 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)
Next – Chapter 7 - The Future of SCSI
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