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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).

What is SCSI?

General SCSI Specifications

What is an SCA / RAID connector?

Types of SCSI Signals

Rules for Connecting SCSI Devices

Back to SCSI School Main Page

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