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This guide provides an insight into configuring a system to provide termination on a SCSI bus.
When the SCSI bus is properly terminated the first device and the last device on the SCSI bus must be terminated. Termination is removed or disabled on any device in between "first and last" as shown in the diagrams below.
Some SCSI host adapters can support up to six other SCSI devices. These devices can be linked directly to the host adapter or linked to each other in a daisy chain.

scsi_drawing1The diagram to the left shows an example of how SCSI bus termination is configured on a host adapter Adaptec 1505 SCSI controller card with a single internal SCSI Tape Drive. The SCSI adapter is first in the chain and so is terminated, and the Tape Drive is last in the chain and therefore is also terminated. Both the SCSI adapter and the SCSI Tape Drive are supplied and fitted with terminating resistors as default.

The diagram below shows how a host adapter is configured when linked to a mixture of three SCSI devices (both internal and external) which are linked serially.


First in the chain is an internal SCSI hard disk which is terminated, this is connected to the SCSI adapter via it's internal interface. The SCSI adapter has an external tape drive linked to it's external interface and so is not terminated. The tape drive is linked to an external SCSI hard disk which is also not terminated. Last in the chain is the external SCSI hard disk which is terminated. The line the devices share is called the SCSI bus.

Why Terminate?
SCSI is a very powerful I/O protocol that handles large amounts of data very quickly. Disturbances on the SCSI bus - such as signal distortion, or echo, can cause problems. Terminating the SCSI bus filters out signal distortion. It is therefore very important that the bus be properly terminated.

SCSI DAT drives.

On a DAT drive, SCSI termination is either enabled or disabled by installing or removing resistor packs. Power to the terminators is supplied by one SCSI terminator power jumper/switch. Craystone despatch DAT drives with termination enabled. To disable or remove termination, please refer to the various set up guides available within the Technical Information section.