Siemens SIMATIC ST-1500 Function Manual page 43

The basics of counting, measurement and position detection
2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
Enable by a digital input
The figure below shows an example for periodic synchronization at signal N depending on
the status of a digital input:
As long as synchronization for counting up is enabled and the corresponding digital input is
active, the counter is synchronized at each signal N. If one of the conditions is not met, the
counter is not synchronized at the signal N.
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Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG