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GFK-1527A Chapter 5 Fault Detection 5-11
5
Maintaining Parallel Bus Termination
It is important when doing online repair to maintain parallel bus termination on the active unit.
This is the reason a terminated parallel cable (IC697CBL803, IC697CBL811 or IC697CBL826) is
used, and why the Redundancy Communications Module
must
be the last device on the parallel
bus. The terminated end of the cable may be safely removed from a de-energized RCM. The
terminated cable should be considered an integral part of the unit it terminates.
On-Line Repair Recommendations
It is advised when doing online repair to power-off the entire PLC system (of the suspect unit),
including ALL RACKS. Change the suspect part, and power-up in STOP mode. Verify that the
links are operational before switching to RUN (%S bits and RCM LEDs are updated in STOP
mode).
Power Supply
The power supply has adequate internal fault detection, which causes it to automatically shut down
if there is a failure.
A power supply failure is indicated by the absence of the OK indication at the power supply. There
are a small number of failures that can result in a false indication or no indication. The probability
of these occurring are extremely low compared to the major failure items of the power supply.
In the event of a power supply failure, the backup CPU takes control of the system. The power
supply can be replaced with power removed from its rack without interruption to the application
being controlled.
Caution
Before replacing a power supply, be sure to disconnect main power to the rack, since
incoming power will be present on the power supply terminals
When the power supply is replaced, power can be returned to the rack. The CPU will then obtain
synchronization with the active system and either take control or become the backup CPU.
Racks
The only detectable rack failure is bad data across the backplane. This bad data can take the form of
a bad control line as well as a bad data or address line. In most cases bad data lines are detected by
the data integrity checks associated with the data transfers. If these occur the PLC is faulted and
control transfers to the backup unit. An indication is given that a data transfer error has occurred.
There is no single indication that a rack failure has occurred. The rack is a very reliable component
in the system and rack failures are extremely rare. A rack failure (other than a catastrophic rack
failure) can only be correctly diagnosed by process of elimination.