Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 86)

Programming Techniques
Chapter 5
5-5
The force table is not transferred from the primary to the backup processor
over any communication channel. Thus, if forces have been set in the
primary processor, they will not be transferred to the backup processor. If
switchover occurs, the backup processor will become active but no forces
will be set.
To prevent this undesirable situation, we suggest that you set the forces
first in the primary processor and then in the backup processor. When it is
necessary to remove forces, we suggest that you remove forces from the
backup processor before removing them from the primary processor.
If switchover occurs while a block transfer is in progress, the transfer may
abort prematurely.
To ensure the integrity of block transfers, you must enter the block transfer
instruction(s) in both the primary and backup processors. Then, the ladder
program in the backup processor should reinitialize the block transfer after
switchover.
You should use memory communication modules or a backup
communication channel to transfer block data transfers from the primary to
backup processors. For more information about system configuration, refer
to chapter 1 in this manual.
When you restart a repaired processor, you must update its internal storage
areas. These are the areas that store dynamic variables, for example, the
values of counters and timers.
To update the internal storage areas, perform data transfers between the
working and repaired processors using either memory communication
modules or a backup communication channel.
Forcing I/O
Executing Block Transfers
Restarting a Repaired
Processor