1. HOW DO I TERMINATE A SCSI BUS?
When a SCSI controller issues a command to one of the devices on the SCSI bus, it does not expect to have that command come back to it. Instead, it expects to hear just the response from the drive it issued a command to. Properly terminating the SCSI bus prevents signal reflection on the cables, essentially absorbing commands when they reach the end of the cable. On an improperly terminated SCSI bus, it is common to have the controller software issue error messages caused by unexpected activities on the bus or to experience complete failure of the bus.
The "golden" rule for SCSI termination is that each end of a SCSI bus must be terminated and no terminator can exist anywhere in between these ends. When you add or remove drives from a SCSI bus, you must comply with this rule.
Each SCSI bus starts with a controller. The controller is terminated. Even if there are no drives attached to it, the controller can still issue commands to search for drives without having those commands reflect. Typically, you will add at least one SCSI drive to the bus. You will do so by connecting a ribbon cable from the controller to the drive. Note that one end of the ribbon must plug into the controller and the other end must plug into the drive. In this case, the drive must also be terminated so that both ends of the bus are terminated according to the rule! If you then add additional drives, you can install the drives anywhere along the cable, providing only the drive on the end is terminated.
The termination rule gets complicated when you try to add both internal and external SCSI drives to a system. Since the rule states that the ends of the SCSI bus must be terminated, obviously the drive connected to the end of the internal cable must be terminated and the drive connected to the end of the external cable must be terminated. Unfortunately, this leaves the controller somewhere in the middle of the bus and it too is terminated. If the controller does not provide automatic termination enable/disable, you must manually disable termination on the SCSI controller.
Note that termination violations might sometimes appear to work. However, it is better to be safe than sorry. Follow the rule!
Terminating a drive usually involves installing or removing a jumper on the drive. Check your drive manual for this jumper setting. If the drive does not provide built-in termination, you can purchase a stand-alone terminator. Terminators are available to plug into most SCSI connectors and come in a whole host of shapes and sizes so get the one designed for your application. Note that most new SCSI drives do not provide a built-in terminator and termination is provide by a separate terminator plugged into the end of the SCSI cable.
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