Termination

A SCSI chain must link devices in a linear arrangement; there can be no fork in the SCSI chain. The device on each end of the chain must be terminated, meaning that it must have a set of resistors attached to it to stop the SCSI signals from echoing back and forth across the SCSI chain. Without terminators, a SCSI bus tends to become unreliable.

Figure 9.5 shows two types of SCSI terminators. The comb-like objects on the right are terminating resistor packs for an old SCSI device. The terminators are inserted into sockets on the SCSI device to terminate it, and removed to unterminate the device. You won't see such resistor packs on most modern SCSI devices, however. Today's models incorporate termination that can be enabled or disabled by setting a jumper.

External SCSI devices are sometimes terminated by a switch or slider, similar in principle to a jumper on an internal device. Other times, it's necessary to use an external SCSI terminator on the end of the chain. The object on the left in Figure 9.5 is such a device. It attaches to one of the SCSI connectors on an external SCSI device, thus terminating it.

Chapter 9

Figure 9.5

SCSI terminators, like SCSI cables, come in a variety of shapes and sizes.

Figure 9.5

SCSI terminators, like SCSI cables, come in a variety of shapes and sizes.

SCSI terminators come in three varieties: passive, active, and low-voltage differential (LVD). Each is more advanced than the one before. Passive terminators are acceptable for low-end SCSI-1 devices, but most more advanced SCSI devices require at least active termination. Ultra2 and Ultra3 SCSI devices require LVD terminators when they're run at Ultra2 speed or above.

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