Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 15)

Manage tasks Chapter 1
The following guidelines generally leave enough time for unscheduled
communication.
• Adjust the period of the tasks as needed to get the best balance between
running your logic and servicing unscheduled communication.
• If your project has a continuous task, unscheduled communication occurs as
a percentage of controller time (excluding the time for periodic or event
tasks).
An overlap is a condition where a task (periodic or event) is triggered while the
task is still running from the previous trigger.
Important: If an overlap occurs, the controller disregards the trigger that caused the overlap. In other words, you might
miss an important execution of the task.
Description
Task trigger occurs.
Task runs.
Task trigger occurs.
Task runs.
Task trigger occurs.
Task runs.
Overlap occurs. Task is triggered while it is still running.
The trigger does not restart the task. The trigger is ignored.
Each task requires enough time to finish before it is triggered again. Make sure that
the scan time of the task is significantly less than the rate at which the trigger
occurs. If an overlap occurs, reduce the frequency at which you trigger the task.
If the type of task is Then
Periodic Increase the period of the task.
Event Adjust the configuration of your system to trigger the task less frequently.
Avoid overlaps
Rockwell Automation Publication 1756-PM005-EN-P - October 2014 15