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SCSI Bus Termination
Termination113 is used to eliminate signal reflections along the
electrical path of the SCSI bus. Any part of the SCSI bus electrical
path that is different from the rest of the path can cause an electrical
reflection. These signal re-flections can combine or interact in ways
to cause corrupted commands or data. Signal reflections can result
in erratic operation or data loss.
Primary Rule of SCSI Termination
The primary rule of termination is that a SCSI bus must be terminated at two and only two points. Both of these points must be at the physical ends of the bus.
There are four types of termination
for a SCSI bus:
- Passive Termination:114 This consists of resistors only.
- Active Termination:115 This type adds a voltage regulator to ensure that
the SCSI signals are always terminated to the correct voltage levels.
Active ter-mination is recommended when the bus is near the maximum
length and when SCSI bus speeds are greater than 10 MHz.
- Forced Perfect Termination (FPT):116 This is similar to active termination but uses diode
clamps to eliminate overshoot and undershoot of signal voltage. Forced
Perfect termination should not be used for a differential bus.
- Multimode Termination: Low-Voltage Differential
(LVD) buses require the use of a modified type of termination called
multimode. Multimode termination allows single-ended
and LVD devices to coexist on the same physical bus. Multimode terminators
sense what type of device is being used and terminate the bus accordingly.
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