Allen-Bradley 1756-CNB/E, User Manual

Page: 83 / 186

Publication 1756-UM523F-EN-P - December 2006
Configure and Program the Controller 83
Interrupted Switchover
A. This data is sent to the secondary controller:
•
Data from the program in the higher priority task.
•
Data from the first part of the program in the lower priority
task.
B. Execution returns to the program in the lower priority task.
C. The secondary controller:
•
starts the scan at the beginning of the program that was in
progress in the primary controller during the switchover.
•
uses the data from the last update.
In this example, the secondary controller starts the scan with an
image of the data as it was during the primary controller’s last
scan.
To prevent a scan from repeating after a switchover:
•
look for array shift instructions.
•
look for scan-dependent logic.
•
take preventative actions
Switchover
occurs.
•
This portion of the task is not executed
during this scan.
•
Instructions executing at the time of the
switchover do not complete in this scan.
Primary Controller
Secondary Controller
Interruption by the program
in a higher priority task.
Start of the program in the lower priority
task (for example, the continuous task).
End of the program in a
higher priority task.
A.
B.
C.