Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 87)

Programming Techniques
Chapter 5
5-6
A PLC-3 backup system using memory communication modules requires
the least amount of communication overhead. Data is transferred quickly
and efficiently from the memory communication module in the primary
processor to its partner in the backup processor over a dedicated parallel
communication link.
In PLC-3 scanner backup systems, the only practical method of getting
data from the primary to the backup processor is by using a backup
communication channel or Data Highway/Data Highway Plus
communication. In some applications, it may be possible to reduce the
amount of data being transferred across the backup communication
channel. This provides faster backup communication updates.
Figure 5.3 illustrates one technique that could eliminate the need to
transfer block-transfer inputs to the backup processor. This program
minimizes the number of unnecessary block transfers. Block transfer write
operations are performed only if the done bit of the associated block
transfer read operation is enabled. The run/backup bit is used to keep the
done bit off in the backup processor. This technique can eliminate the need
to use backup communication for block transfer inputs if the following
conditions are true:
only block transfer outputs depend on the block transfer input
conditions
the block transfer outputs can withstand a one-time update time (at
switchover) of twice the normal update time plus 100 milliseconds
all operations using the block transfer input data require only current
values of the input data
If your system does not meet these conditions, use backup communication
to transfer the block transfer input data to the backup processor.
Reducing Backup
Communication Overhead