Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 121)

Publication 1785-UM024B-EN-P - January 2003
Application Guidelines E-9
How to Complete Qualification in the Same Process State
During qualification, the qualifying PLC-5 transitions into Run mode
and begins scanning the program ladder logic.This is when the two
programmable controllers are most likely to get into different process
states.
One approach to avoid this is to have the qualifying PLC-5 track the
process state of the primary PLC-5 until the state crossload is finished.
This should be done with a scheduled message every NUT. You can
hold off qualification completion by setting a word in the CBI file
during the first scan. The application can then check to see when the
state crossload is completed. When both programmable controllers are
in the same process state, the application can clear the qualification
hold-off word in the CBI file and the PLC-5 will qualify as secondary.