Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 88)

Switchover Considerations
Chapter 6
6-3
Using the 1785-BCM module, the primary system can transfer data to the
secondary system at a maximum throughput of 4K words in less than 1
second. This time takes into account:
program scan
programming method used to transfer data to secondary processor
PLC-5 block-transfer queuing
number of remote I/O chassis
number of remote I/O block transfers
For actual data table transfer times when transferring different block
lengths of data, refer to Appendix A “Sample Programs.”
The program scans are not synchronized; therefore, it is possible that the
processors read the same inputs and control outputs accordingly, but at
slightly different times. Sometimes, the processors will read different
input values and make different decisions. This means that the internal
states of the two systems may diverge.
If the two systems diverge, the secondary system generates a bump when it
takes control of the process. Like data table transfer time, the duration of
this bump is a function of how fast the inputs of the machine or process are
changing, as well as the amount of data to be sent between the two
systems.
To limit divergence, we recommend that the primary and secondary
processors execute the same programs.
Important: When you use outputs (from 0 file) to seal-in a rung with
non-retentive output instructions, the seal-in will be lost if the processors
switch primary and secondary twice, consecutively. This occurs because a
PLC-5 processor, when powering-up, going from PROGRAM to RUN
mode, or clearing a fault, resets outputs and performs a prescan to force all
rungs false before going into RUN mode. This turns off all output bits
controlled by non-retentive output instructions. This will cause divergence
between the primary and secondary processors.
Divergence