Siemens SIMATIC ST-1500 Function Manual page 24

2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
Capture (Latch)
You can configure an external reference signal edge that triggers the saving of the current
counter value or position value as a Capture value. The following external signals can trigger
the Capture function:
● Rising or falling edge of a digital input
● Both edges of a digital input
● Rising edge of the N signal at the encoder input
When using a digital input, you can specify whether counting is to continue from the current
counter value or from the start value after the Capture function.
Synchronization
You can configure an external reference signal edge to load the counter with the specified
start value. The following external signals can load the counter with the start value:
● Rising or falling edge of a digital input
● Rising edge of signal N at the encoder input
● Rising edge of signal N at the encoder input depending on the level of the assigned
digital input
Hysteresis
You can specify hysteresis for the comparison values, within which a digital output is
prevented from switching again. An encoder can come to a standstill at a specific position,
and slight movements may make the counter value fluctuate around this position. If a
comparison value or a counting limit lies within this fluctuation range, the corresponding
digital output will be switched on and off with corresponding frequency if hysteresis is not
used. The hysteresis prevents these unwanted switching operations.
Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG
The basics of counting, measurement and position detection
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