Specifications

SIMATIC Instructions
9-50
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
You can change the characteristics of a PTO or PWM waveform by modifying the
desired locations in the SM area (including the control byte), and then executing
the PLS instruction.
You can disable the generation of a PTO or PWM waveform at any time by writing
zero to the PTO/PWM enable bit of the control byte (SM67.7 or SM77.7), and then
executing the PLS instruction.
Note
Default values for all control bits, cycle time, pulse width, and pulse count values
are zero.
Note
The PTO/PWM outputs must have a minimum load of at least 10% of rated load to
provide crisp transitions from off to on, and from on to off.
PWM Operation
The PWM function provides for variable duty cycle output. The cycle time and the
pulse width can be specified in a time base of either microseconds or milliseconds.
The cycle time has a range either from 50 microseconds to 65,535 microseconds,
or from 2 milliseconds to 65,535 milliseconds. The pulse width time has a range
either from 0 microseconds to 65,535 microseconds, or from 0 milliseconds to
65,535 milliseconds. When the pulse width is specified to have a value greater or
equal to the cycle time value, the duty cycle of the waveform is 100% and the
output is turned on continuously. When the pulse width is specified as 0, the duty
cycle of the waveform is 0% and the output is turned off. If a cycle time of less than
two time units is specified, the cycle time defaults to two time units.
There are two different ways to change the characteristics of a PWM waveform:
with a synchronous update and with an asynchronous update.
 Synchronous Update: If no time base changes are required, then a
synchronous update can be performed. With a synchronous update, the change
in the waveform characteristics occurs on a cycle boundary, providing for a
smooth transition.
 Asynchronous Update: Typically with PWM operation, the pulse width is varied
while the cycle time remains constant. Therefore, time base changes are not
required. However, if a change in the time base of the PTO/PWM generator is
required, then an asynchronous update is used. An asynchronous update
causes the PTO/PWM generator to be disabled momentarily, asynchronous to
the PWM waveform. This can cause undesirable jitter in the controlled device.
For that reason, synchronous PWM updates are recommended. Choose a time
base that will work for all of your anticipated cycle time values.