The Basics of SCSI Termination

The Basics Computer System Termination

Since its inception over 10 years ago the SCSI (Small Computer System Interface) has evolved into
a popular industry standard. With this evolution, performance has been improved by a factor of over
ten fold. Data transfer rates now approach 40 MBytes/Second, but with this increase in performance
it becomes crucial to use only the best quality cables and terminators.

The SCSI Bus is designed to have both ends physically terminated. Diagram 1 shows a standard
SCSI configuration.

Diagram 1

The Basics of SCSI Termination

The SCSI specification states that you can have one host and up to 7 targets connected to a SCSI
Bus. You must terminate both the host and the last target device on the daisy chain.The cables
should not have a total length of over 20 feet. Use the shortest cables available to keep this distance
less than 20 feet.

Each SCSI device attached to the SCSI daisy chain must have a different address. The host adapter
will usually occupy address 7, leaving address 0 through 6 for the attached target devices. With an
external drive the SCSI address can usually be set on the back of the drive subsystem, or if you
are using an internal drive, refer to the owners manual for the jumper settings.

Some systems require a target device to be at a certain address in order to work correctly.This is
common with drives that are to be boot devices. Most host adapters require them to be set at
address 0. Some UNIX systems have special addresses set for Tape Drives and CD-ROMs.
Refer to their individual manuals for details.

It should also be noted that most internal drive mechanisms come with internal terminators already
installed. These terminators take the form of 2 or 3 single in-line resistor paks (See Diagram 2).
These terminators are socketed and easily removed. The color of these resister paks is usually,
black, red, blue, or yellow.

Diagram 2

The Basics of SCSI Termination Diagrams