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What kind of termination do I need on my SCSI bus?
(KB # F0439E58)

SCSI bus communication is accomplished by sending electrical pulses down a wire. All wires have an impedance (resistance) to sending signals down the SCSI bus. Most SCSI cables have an impedance of around 100 ohms. In a non-terminated bus the signals go out to open air which has exceptionally high impedance (infinite). Because of this, the signals hit a "wall of impedance" and are echoed (reflected) back down the SCSI bus. Reflected signals will cause line noise problems and possible data errors. To keep this from happening, terminators are placed at both physical ends of the SCSI bus. The terminator's job is to closely match the impedance of the SCSI bus cable to prevent the signals from reflecting back down the cable.

Termination is required at both physical ends of the SCSI bus. Many SCSI controllers automatically enable termination at the controller itself when it is at one physical end of the bus. However, it may be necessary for you to enable or disable termination at the controller. Check your SCSI controller documentation for more information regarding your controller. In any case, it is necessary to install a terminator at the last device on the SCSI bus.

Note: Do not install a terminator in the middle of the SCSI bus. Any devices attached beyond the terminator may not communicate with the controller.

The type of terminator you use depends on the interface type of your SCSI bus:

Single-ended: Single-ended terminators are referred to as passive or active. For Fast or Ultra single-ended buses, active termination is recommended for stable operation.

Differential (HVD): A High-Voltage Differential bus requires an HVD terminator. Do not use a single-ended or LVD terminator on an HVD bus.

Low-Voltage Differential (LVD): In order for a SCSI bus to operate in LVD mode, all devices, the SCSI controller, and the terminator must be LVD. Many devices and terminators are "multimode ", capable of operating in either LVD or single-ended mode.



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