Siemens SIMATIC ST-1500 Function Manual page 37

The basics of counting, measurement and position detection
2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
Single synchronization
The figure below shows an example of the EN_SYNC_UP, EN_SYNC_DNand
EVENT_SYNC bits with single synchronization by an edge at a digital input for count pulses
in an upwards direction:
After synchronization is enabled for counting in an upwards direction, the counter is
synchronized at the first rising edge at the configured digital input. The counter can only be
synchronized again once the control bit (Page 185) EN_SYNC_UP has been reset and set
again.
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Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG