Siemens SIMATIC S7 System Manual page 288

Program instructions
7.7 Pulse output
Single-Segment pipelining of PTO pulses
In single-segment pipelining, you are responsible for updating the SM locations for the next
pulse train. After the initial PTO segment has started, you must immediately modify the SM
locations with the parameters of the second waveform. After you update the SM values,
execute the PLS instruction again. The PTO function holds the attributes of the second pulse
train in a pipeline until it completes the first pulse train. The PTO function can store only one
entry at a time in the pipeline. When the first pulse train completes, the output of the second
waveform begins, and you can store a new pulse train specification in the pipeline. You can
then repeat this process to set up the characteristics of the next pulse train. Attempting to
load the pipeline while it is still full results in the PTO Overflow bit (SM66.6, SM76.6, or
SM566.6) being set and the instruction being ignored.
Smooth transitions between pulse trains occur unless the active pulse train completes before
a new pulse train setup is captured by the execution of the PLS instruction.
Note
In single-segment pipelining, the frequency has an upper limit of 65,535 Hz. If a higher
frequency is needed (up to 100,000 Hz), multiple-segment pipelining must be used.
Multiple-Segment pipelining of PTO pulses
In multiple-segment pipelining, the S7-200 SMART automatically reads the characteristics of
each pulse train segment from a profile table located in V memory. The SM locations used in
this mode are the control byte, the status byte, and the starting V memory offset of the profile
table (SMW168, SMW178, or SMW578). Execution of the PLS instruction starts the multiple
segment operation.
Each segment entry is 12 bytes in length and is composed of a 32 bit starting frequency, a
32 bit ending frequency, and a 32-bit pulse count value. The table below shows the format of
the profile table configured in V memory.
The PTO generator automatically increases or decreases the frequency linearly from the
starting frequency to the ending frequency. The frequency is increased or decreased by a
constant value at a constant rate. Once the number of pulses reaches the specified pulse
count, the next PTO segment is loaded. This sequence repeats until it reaches the end of the
profile. A segment's time duration should be greater than 500 microseconds. If the time
duration is too small, the CPU may not have enough time to calculate the next PTO segment
values. If the next segment cannot be calculated in time, then the PTO pipeline underflow bit
(SM66.6, SM76.6, and SM566.6) is set to "1" and the PTO operation terminated.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART