USPIIe-USB Hardware Manual
1-4
Themis Computer
1.2.2
SCSI Termination
All SCSI busses must be properly terminated. The USPII
e
-USB contains two SCSI
ports (A and B) on the Front Panel of the Baseboard (see Appendix C, “Front Panel
Diagrams”). The SCSI A bus is accessed through the Front Panel and the VME P2
connector (through a Paddle Board) behind the VME backplane (see
Figure 1-1
on
page 1-5).
The SCSI B bus is accessed through the Front Panel only. As a result, the
USPII
e
-USB SCSI Controller is always at one end of the SCSI B bus, hence the on-
board SCSI B terminator is always enabled. The SCSI A bus, on the other hand, can
have several configurations, which determine whether or not the bus is terminated at
the USPII
e
-USB. Possible SCSI A bus configurations are shown in
Figure 1-2
on
page 1-6, along with the SCSI B bus flow path.
Caution:
SCSI busses must be properly terminated in order for SCSI devices to
operate correctly.
Only one device (including the SCSI Controller) at each end of a SCSI bus chain
should be terminated. If both the Front Panel and the Paddle Board have SCSI-A
devices attached, then the on-board SCSI-A terminator should be
disabled
(Off);
otherwise, the on-board SCSI-A termination should be
enabled
(On).
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