98 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 5 Fast Module Features
Figure 13 compares two applications in which the output is instructed to turn
On for 4.5 seconds:
• In the application without PWM, a single pulse is generated. The pulse
remains active for the same length of time the Data output tag is On
(4.5 seconds).
• In the application with PWM, a series of pulses are generated. Each pulse is
active for a configured On time of .5 seconds or 50% of the 1 second cycle
time. The Data output tag is On for 4.5 seconds.
Figure 13 - PWM
By default, PWM is configured to continue the output pulse train until the
output logic turns Off. When the output logic turns Off, the output pulse train
immediately stops.
Figure 14 - PWM with Truncated Pulse
Output Logic
Output State
Output logic is On for 4.5 seconds.
Output is active for 4.5 seconds.
Output Logic
Output State
Output logic is On for 4.5 seconds.
Each pulse is active for 0.5 seconds (On time)
Application without PWM
Application with PWM
1 Second
Cycle Time
In Figure 14, the output logic is On for 4.25 seconds and then turns Off in the
middle of the last pulse. Even though the PWM On time is configured for 0.5
seconds, the last pulse is only active for 0.25 seconds because it is truncated
when the output logic turns Off.
Output Logic
Output State
Output logic is On for 4.25 seconds.
The last pulse is truncated when the output logic turns Off.
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