Performance Computer PT-VME151A User Manual page 33

2
Section GETTING STARTED
are normally target devices. With a single target (disk drive) and a single initiator (the PT-
VME151A) on a SCSIbus, termination is provided by the respective devices at each end of the
cable. If either end were to lose power then termination would be moot because there would be
no data to transfer.
Multiple PT-VME151As have been tied together on a single SCSIbus to use the SCSIbus as a
very high performance Local Area Network. In this application it was advantageous to be able
to power down an individual PT-VME151A without having to worry about whether it was the
one supplying the SCSIbus termination. The SCSI TERMPWR mechanism provided the solu-
tion.
The PT-VME151A's +5V supply goes through fuse F1 then diode CR2 to be OR'ed into the
SCSI TERMPWR signal. This allows the PT-VME151A to supply SCSIbus termination power
when it is powered up and some other device on the SCSIbus to provide it when it is not. The
fuse protects the PT-VME151A's power supply from short circuit conditions on the
TERMPWR signal.
If for some reason you do not want the PT-VME151A to contribute to TERMPWR remove fuse
F1. Fuse F1 is removed by pulling it to the right as shown by the bidirectional arrow on top of
it in Figure 6.
NOTE: If unreliable SCSI operation occurs, the fuse, terminator resistor packs and jumper
settings should be checked.
18 Extensible Single Board Computer/Controller User's Manual
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