Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 53)

Manage event tasks Chapter 2
When the axis reaches the position that is specified as the watch position, it
triggers the event task.
• You must first arm the axis for the watch position by using a Motion Arm
Watch (MAW) instruction.
• In the MAW instruction, the Trigger Condition operand defines the
direction in which the axis must be moving to trigger the event task.
• Once the axis reaches the watch position and triggers the event task, you
have to re-arm the axis for the next watch position.
This timing diagram shows the relationship between the watch position and the
event task.
Checklist for an axis watch task
For This Make Sure You
1. Watch position Use a MAW instruction to set up a watch position. This lets the axis trigger the event task when it reaches the watch position.
•
Initially, arm the axis to detect the first watch position.
• When the axis reaches the watch position and triggers the event task, re-arm the axis for the next watch position.
• Re-arm the axis fast enough to detect each watch position.
If your normal logic is Then
Fast enough to re-arm the axis between intervals of the watch position
(For example, normal logic always completes at least two scans
between watch positions.) Arm the axis within your normal logic, if desired.
Not fast enough to re-arm the axis Arm the axis within the event task.
2. Task priority Configure the event task as the highest priority task.
If a periodic task has a higher priority, the event task may have to wait until the periodic task is finished.
3. Number of event tasks
Limit the number of event tasks.
Each additional task reduces the processing time that is available for other tasks. This could cause an overlap.
4. Automatic output processing
For an event task, you can typically disable automatic output processing (default). This reduces the elapsed time of the task.
Rockwell Automation Publication 1756-PM005-EN-P - October 2014 53