Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 167
Program the Redundant Controller Chapter 7
Use Periodic Tasks
If you have multiple tasks in your project, changing the System Overhead Time
Slice does not affect how communication is serviced. To increase the time to
service communication when multiple tasks are used, configure the periodic tasks
such that more time might be available to service communication.
If you use periodic tasks, communication is serviced any time a task is not
running. For example, if you configure your task period at 80 ms and the task
executes in 50 ms, the controller has 30 ms out of every 80 ms to service
communication.
Figure 50 - Periodic Task Execution and Service Communication
If you use multiple periodic tasks, verify the following:
• The execution time of a highest priority task is significantly smaller than
its period.
• The total execution time of all your tasks is significantly less than the
period of the lowest priority tasks.
Verifying those settings generally leaves enough time for service communication.
The example configuration of tasks shown here demonstrates those
configuration settings.
While you can use multiple periodic tasks in your
redundant controller program, use the fewest number of
tasks possible.
50 ms 50 ms 50 ms
Task Execution
30 ms 30 ms 30 ms
Service Communication
Periodic Task
Periodic Task Periodic Task
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