Allen-Bradley 1756-CNB/E, User Manual

Page: 122 / 186

Publication 1756-UM523F-EN-P - December 2006
122 Maintain and Troubleshoot the System
Make All Your Tasks Periodic
Action Details
1. If you have more than one task, make
them all periodic tasks.
If the controller contains only a periodic task or tasks, the system overhead time slice value
has no effect. Unscheduled communication happens whenever a periodic task is not
running.
Example
Suppose your task takes 50 ms to execute and you configure its period to 80 ms. In that
case, the controller has 30 ms out of every 80 ms for unscheduled communication.
2. Follow these guidelines to set the
periods of the tasks.
If you have multiple tasks, make sure that:
•
The execution time of a highest priority task is significantly less than its period.
•
The total execution time of all your tasks is significantly less than the period of the
lowest priority tasks.
This generally leaves enough time for unscheduled communication.
For example, in this configuration of tasks:
•
The execution time of the highest priority task (Task 1) is significantly less than its
period (20 ms is less than 80 ms).
•
The total execution time of all tasks is significantly less than the period of the lowest
priority task (50 ms is less than 100 ms).
3. Tune the periods of the tasks. Adjust the periods of the tasks as needed to get the best trade-off between executing your
logic and servicing unscheduled communication.
4. Look for overlaps. Look at the Monitor tab of the properties of the task to see if overlaps are happening. An
overlap happens if the period of a task is less than its scan time. If you see overlaps,
increase the period of the task.
50 ms 50 ms 50 ms
Periodic Task
30 ms 30 ms 30 ms
Unscheduled
Communication
Task Priority Execution Time Rate
1 Higher 20 ms 80 ms
2 Lower 30 ms 100 ms
Total execution time: 50 ms