Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules User Manual
Page 99
Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
99
Fast Module Features
Chapter 5
You can modify the default PWM configuration for each of a module’s 16
outputs for further control of an output’s pulse train, as described in
. Configuration options include the following:
•
Cycle Limit and Execute All Cycles, as described below
•
Minimum On Time, Extend Cycle, and Stagger Output as described on
Cycle Limit and Execute All Cycles
You can limit the number of pulse cycles that occur while an output is On. This
feature is useful when you want to apply a level of output control when a process
is stopped. For example, in a gluing application, you may want to apply 4 drops of
glue to a product when the product is within a fixed window on a conveyor belt.
By configuring a cycle limit of 4, you can make sure that only 4 drops of glue are
applied even if the conveyor belt stops with the product in the window.
Controlling the process with the Cycle Limit feature eliminates the need to write
complex logic to detect a stopped conveyor belt.
Figure 15
shows a PWM pulse train configured with a cycle limit of 2. The
PWMCycleLimitDone input tag indicates when the PWM cycle limit has been
reached. The corresponding bit is reset upon the next rising edge of the output
which restarts PWM.
Figure 15 - PWM Cycle Limit
IMPORTANT
The Program and Fault mode states configured for the module override the
PWM output state unless the point is configured to hold the last state while in
Program or Fault mode. If a point is configured to hold the last state and the
output is currently On, the output will continue to use PWM until the PWM
cycle limit is reached, the module transitions out of Program or Fault mode, or
a final fault state goes into effect.
For more information, see the following:
•
Configurable Point-level Output States on page 55
•
Programmable Fault State Delays on page 95
•
Cycle Limit and Execute All Cycles on page 99
Output Logic
Only 2 cycles are executed even
though the output logic remains On.
Output State
The cycle limit restarts when the output begins pulsing
on the next rising edge of output logic.