Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 87)

Switchover Considerations
Chapter 6
6-2
Input Signal Update Time
Immediately after the switchover, it is possible that the secondary
processor will be reading different input conditions. To avoid a sudden
change in the outputs during switchover, all of the input signals must be
present for at least a time equal to the sum of:
program scan
I/O scan
input module delay
1785-BCM switchover time
If these conditions are not met, outputs may be assigned different states in
the two processors. This can cause a sudden change of operation when
switchover occurs.
Time-out on Remote I/O Link
The remote I/O chassis are updated one at a time. The total remote I/O
update is 10 ms per chassis (typical).
To keep the I/O chassis from faulting and losing control of the I/O, the
switchover in the remote I/O link (the time from when the remote I/O
relays in the primary 1785-BCM module open to when the remote I/O
relays in the secondary 1785-BCM module close) will be accomplished in
less than 50 ms. (The 1785-BCM module will do this automatically.)
This switchover time is less than the 100 ms for remote I/O adapter
watchdog timeout; therefore it allows continuous control of the I/O from
the backup system processors.
Data Table Transfer Time
This time refers to the time it takes to transfer critical data from the
primary system to the secondary system. Data table transfer time is
dependent on the amount of data to be sent between the two systems, the
number of remote I/O chassis, and the number of remote I/O block
transfers that are being executed.