Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 14)

Chapter 1 Manage tasks
Description
Initially, the controller runs the motion planner and the I/O task (if one exists).
After running the continuous task for 4 ms, the controller triggers the system overhead.
The period for periodic task 1 expires (12 ms), so the task interrupts the continuous task.
After running the continuous task again for 4 ms, the controller triggers the system overhead.
The trigger occurs for event task 1.
Event task 1 waits until the motion planner is done.
Lower priority tasks are delayed.
The continuous task automatically restarts.
The Studio 5000 environment includes a task monitor tool on the distribution
CD. You can use this tool to analyze how tasks are running.
Unscheduled communication occurs only when a periodic or event task is not
running. If you use multiple tasks, make sure that the scan times and execution
intervals leave enough time for unscheduled communication. Use the following
methods to plan enough unscheduled communication time.
1. Verify that the execution time of a highest priority task is significantly less
than its specified period.
2. Verify that the total execution time of all your tasks is significantly less than
the period of the lowest priority tasks.
For example, the following is true in this configuration.
Task Priority Execution Time Period Specified
1 Higher 20 ms 80 ms
2 Lower 30 ms 100 ms
Total execution time: 50 ms
• The execution time of the highest priority task (Task 1) is significantly less
than its specified period (20 ms is less than 80 ms).
• The total execution time of all tasks is significantly less than the specified
period of the lowest priority task (50 ms is less than 100 ms).
Leave enough time for
unscheduled
communication
14 Rockwell Automation Publication 1756-PM005-EN-P - October 2014