SCSI termination Q&A

SCSI termination is an important property of a SCSI bus.

Termination means that special electrical resistors (terminators) are installed in the devices at each end of the bus and are not installed in other devices on the bus. With some SCSI devices you must manually insert or remove the terminators. Other devices have built-in terminators that are enabled or disabled via switches or software commands.

Your host adapter and the SCSI devices attached to it must be properly terminated, or they will not work reliably.

Why Is Termination Necessary?

The SCSI bus must be properly terminated at both ends so that commands and datacan be transmitted to and from all devices on the bus. This is roughly similar to sending vibrations back and forth over the length of a string that is firmly secured at both ends.

If you hold the middle of the string, the vibrations cannot be transmitted. This is similar to what happens to data and commands on the SCSI bus if you terminate a device in the middle of the bus.

Host Adapter Termination

This information applies if your computer has a host adapter board. If you use SCSI in a laptop or notebook computer or if a host adapter chip is installed on your computer's motherboard, your host adapter device is automatically terminated, and you do not need to change its termination status. Since there is no room to fit internal SCSI devices inside a laptop or notebook computer, you can only connect external SCSI devices to the host adapter device. This means that the host adapter device is always at the end of the SCSI bus and should always be terminated.
Note : The host adapter device could be a SCSI host adapter chip, a PCMCIA host adapter such as Adaptec's SlimSCSI product, or a parallel port SCSI host adapter such as Adaptec's MiniSCSI product.

Termination is usually enabled by default on host adapters. You don't need to change the default if your computer has only internal SCSI devices or only external SCSI devices. In fact, you may not need to be concerned about host adapter termination at all, since many Adaptec host adapter boards can automatically adjust their termination for any configuration of devices.

However,you need to disable termination on your host adapter if the host adapter board doesn't automatically adjust its termination and you connect both internal and external SCSI devices to it.
See your host adapter documentation to learn how to change the termination setting.

Terminating Other SCSI Devices

Disk drives, CD-ROM drives, tape drives, and other SCSI devices must be terminated if they are the last device at either end of the SCSI bus. Termination must be disabled if a device is in the middle of the bus.

There are several different ways to change termination on SCSI devices:

  • Physically remove terminating resistors from their sockets, or insert them in the sockets (for internal SCSI devices)
  • Change a switch setting on the device's switch block
  • Remove or install a terminator plug (for external SCSI devices)
  • Read the device documentation to find out how to change termination on your SCSI devices.