Specifications

Page 4 DocSTAR Level 2 Service Training Workbook
*Note: Whenever a change is made to the SCSI Bus, all SCSI devices as well as the
CPU must be re-booted. All SCSI devices must be turned on before the CPU is
powered on.
The SCSI cable, which transfers the data from the device to the host adapter or another
device, is specified to be a certain length and quality depending on the SCSI
classification. When dealing with maximum cable length specifications, the specification
is the total cable length from the host adapter to the last physical device. See Appendix A
for cable length maximums.
Figure 1.5: SCSI Cable
The SCSI terminator, which stops Signal Bounce (reflections of the signal) from
continuing back and forth on the SCSI bus, must be placed at both ends of the SCSI bus.
When connecting both internal and external devices, the SCSI host adapter acts as a
bridge and should not be terminated. When connecting only internal or only external
SCSI devices, then the SCSI host adapter must be terminated. Under normal
circumstances the ‘Automatic’ setting will terminate the card properly, if that option is
available. When terminating external devices it is recommended that active terminators
be used instead of passive. All DocSTAR devices should be terminated with an
active terminator.
Figure 1.6: Active Terminators
Active vs. Passive Terminators
Passive terminators use resistor voltage dividers to provide termination power. Active
terminators use one or more voltage regulators to produce the termination voltage. In
more understandable terms, the passive terminators allow signals to fluctuate relative to
the TERMPWR (TERMination PoWeR) voltage. This fluctuation is fine for short distance
data transmission, about 2 to 3 feet. However, longer distance requires the more stable
signal that active terminators provide. Currently, active terminators are recommended or
required by manufacturers for all SCSI set-ups. External active terminators usually have a
green LED on them signifying that they are active and/or they may be marked as ‘Active’.
See Figure 1.6.