Siemens SIMATIC ST-1500 Function Manual page 104

Note
For High_Speed_Counter as of V3.0, the following applies:
You can choose "Capture at signal N" only in operating mode "Use count value as
reference":
Frequency
This parameter is used to define the frequency of the following events:
● Synchronization at signal N
● Synchronization as function of a digital input
You can select from the following options:
Option
Once
(default)
Periodic
Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG
Using the High_Speed_Counter technology object
3.5 Configuring the High_Speed_Counter
Meaning
The counter is only set at the first signal N or the first configured
edge of the digital input.
The counter is set at each signal N or each configured edge of the
digital input.
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