Publication 1763-RM001C-EN-P - October 2009
150 Using High-Speed Outputs
PWM Function
The PWM function allows a field device to be controlled by a PWM wave
form. The PWM profile has two primary components:
• Frequency to be generated
• Duty Cycle interval
The PWM instruction, along with the HSC and PTO functions, are different
than all other controller instructions. Their operation is performed by
custom circuitry that runs in parallel with the main system processor. This
is necessary because of the high performance requirements of these
instructions.
The interface to the PWM sub-system is accomplished by scanning a PWM
instruction in the main program file (file number 2), or by scanning a
PWM instruction in any of the subroutine files. A typical operating
sequence of a PWM instruction is as follows:
1. The rung that a PWM instruction is on is solved true (the PWM is
started).
2. A waveform at the specified frequency is produced.
3. The RUN phase is active. A waveform at the specified frequency
with the specified duty cycle is output.
4. The rung that the PWM is on is solved false.
5. The PWM instruction is IDLE.
While the PWM instruction is being executed, status bits and data are
updated as the main controller continues to operate. Because the PWM
instruction is actually being executed by a parallel system, the status bits
and other information are updated each time the PWM instruction is
scanned while it is running. This provides the control program access to
PWM status while it is running.
Pulse Width Modulation
(PWM) Function File
Within the PWM function file are two PWM elements. Each element can
be set to control either output 2 (O0:0/2) or output 3 (O0:0/3). Function
file element PWM:0 is shown below.
TIP
PWM status is only as fresh as the scan time of the
controller. Worst case latency is the maximum scan of
the controller. This condition can be minimized by
placing a PWM instruction in the STI (selectable timed
interrupt) file, or by adding PWM instructions to your
program to increase how often a PWM instruction is
scanned.
efesotomasyon.com - Allen Bradley,Rockwell,plc,servo,drive
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