A General Guide To SCSI

What is SCSI?

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 motherboard.

  • Internal and/or external SCSI devices (such as hard drives, CD-ROM, DVD, scanner).

  • Internal and/or external SCSI cables, SCSI terminators, and SCSI cable adapters.

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 requirements.

There are three distinct types of SCSI signals:

  1. 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.


  2. 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.


  3. Low Voltage 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.

Types of SCSI Connectors

There are various different connector types for each sort of SCSI variant.

All the SCSI connectors can be found if you CLICK HERE

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).