Rockwell Automation Publication 1756-AT017A-EN-P - January 2013
4 Position-based Output Control with the MAOC Instruction
Factors that Impact Schedule Accuracy
The following sections describe factors that can contribute to a loss of accuracy
on the scheduled output modules.
Minimum Pulse Width
A schedule interval is the course update period (CUP) divided by 16. Each
schedule interval can have only one scheduled event within it. If multiple
schedules fall within a single schedule interval, only the last event is evaluated.
The width of a pulse must be longer than a schedule interval. If the pulse width is
shorter than or equal to the schedule interval, it is less than the minimum pulse
width, and a loss of accuracy can occur.
Figure 1 on page 5 shows a minimum repeatable pulse width for a
1756-OB16IEFS module that supports as many as 16 schedules per CUP. The
summary of events is as follows:
•A—The On and Off events are spaced far enough apart that they cannot
fall within the same schedule interval. This meets the minimum pulse
width criteria.
•B—Even though the On event occurs on the edge of the schedule interval,
the Off event is longer than the minimum pulse width and falls into a
different schedule interval.
•C—The On and Off events fall within the same schedule interval and
cancel each other. The output is never energized.
•D—The pulse has the same width as example C, but the On and Off
events fall into separate schedule intervals. In this case, the output is
energized. Because it is not possible to predict where the schedule interval
starts or ends, the output can turn On at times, but not at others—even on
the same machine.
•E—The pulse width is exactly equal to the schedule interval, and the
output is energized as scheduled in most cases. However, in some cases,
output behavior can be unpredictable. Because it is not possible to predict
where the schedule interval starts or ends, the output can turn On at times,
but not at others—even on the same machine.
Topic Page
Minimum Pulse Width 4
Schedule Limitations 5
Position Reference: Actual versus Command 7
Constant versus Changing Velocity 8
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