Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 15)

Backup Concepts for the PLC-5 System
Chapter 1
1-7
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
- change in the primary processor’s mode from:
RUN to PROGRAM (manual switchover)
REM RUN to REM PROG
REM RUN to REM TEST