Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 110)

Programming Techniques
Chapter 7
7-13
When developing a program for the PLC-5 backup system, you must always
consider the following:
non-synchronous I/O scans
execution times of block transfer instructions
time for transfer of data from the primary to the secondary processor
the need to reduce the quantity of data to be transferred, and gathering all
related data into a single block
synchronization of discrete I/O data with regard to program scans even
though remote I/O scans are performed asynchronously
possibility of block transfer data changing during the scan of the program
automatic transfer of the remote inputs, such as single-transfer and remote
block-transfer data from the primary 1785-BCM module to the secondary
1785-BCM module.
the need to disable message instructions in the secondary processor using the
primary/secondary bit (local is bit 6, remote is bit 14)
Summary of Programming
Considerations