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