Siemens SIMATIC ST-1500 Function Manual page 69

The basics of counting, measurement and position detection
2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
24 V and TTL pulse encoder count signals with Up/Down count signal
The Up count signal is connected to terminal A. The Down count signal is connected to
terminal B.
The figure below shows an example of the time profile of the signals of a pulse encoder with
Up/Down count signal and the resulting count pulses:
Sourcing output/sinking output for 24 V counter signals (TM Count)
You can connect the following encoders/sensors to the counter inputs:
● Sourcing output:
The A, B, and N inputs are wired to 24VDC .
● Sinking output:
The A, B, and N inputs are wired to ground M .
● Push-pull (sourcing and sinking output):
The A, B, and N inputs are wired alternately to 24VDC and ground M .
Sourcing output for 24 V counter signals (Compact CPU)
You can connect the sourcing output and push-pull encoders or sensors to the counter
inputs.
Monitoring of the encoder signals (TM Count and TM PosInput)
The signals of push-pull 24 V sensors are monitored for wire breaks by the technology
module. TTL signal are monitored for offset voltage by the technology module.
If you enable the diagnostic interrupt in the device configuration, the technology module
triggers a diagnostic interrupt in the event of encoder signal errors.
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Counting, measurement and position detection
Function Manual, 12/2017, A5E32009889-AG