Switchover
Should a fault occur in the primary processor, control switches to
the secondary system in less than 50 ms (maximum). When a
switchover occurs, the outputs in the remote I/O maintain their
last state until they
come under the control of the secondary processor.
However, keep in mind that the program scans of the two processors
are not synchronized. This means that the secondary processor may
be scanning all, none, or only part of the program (at your
discretion). This manual explains the switchover process, and
provides guidelines for developing programs for your PLC-5 backup
system. (For more information about switchover, refer to Chapter 6,
“Switchover Considerations.”)
Role of the 1785-BCM Series B Module
As an integral part of the backup system, the 1785-BCM modules
enable high speed communication between the two PLC-5 processors,
and permit the secondary processor to assume control of the
process. In addition, the 1785-BCM module provides:
high speed transfer of the data table values from the primary to
the secondary system, to ensure that the secondary system’s data
table is a copy of the primary system’s a buffer of 4K words for
data table values
exchange of information on the status of the primary and secondary
systems
automatic transfer to the secondary system of the remote input and
block transfer read values (analog values, etc.) transfer of
control from the primary processor to the secondary
processor when one of the following conditions occur:
- power failure
- processor fault
- 1785-BCM module fault