Allen-Bradley 1756-CNB/E, User Manual

Page: 85 / 186

Publication 1756-UM523F-EN-P - December 2006
Configure and Program the Controller 85
C. If a higher priority task interrupts the logic, the value of
scan_count.ACC is sent to the secondary controller at the end of
the program in the higher priority task.
D. If a switchover occurs before the EQU instruction, the secondary
controller starts its execution at the beginning of the program.
The EQU instruction misses the last value of scan_count.ACC.
Take Preventative Actions
If logic seems susceptible to an upset during a switchover, either place
susceptible logic in the highest priority task, or, if the logic must
remain in a lower priority task, take one of these actions:
•
Use UID and UIE Instruction Pairs.
•
Buffer critical data.
Place Susceptible Logic in the Highest Priority Task
This prevents the controller from sending any data to the secondary
controller until the program finishes.
If a switchover occurs during the program, the secondary controller
repeats the scan using the same starting data.