Siemens SIMATIC ST-1500 Function Manual page 71

The basics of counting, measurement and position detection
2.2 Basics of counting, measuring and position input (TM Count, TM PosInput, Compact CPU)
Count signals of RS422 pulse encoders without/with direction signal
The encoder, for example a light barrier, only returns a count signal that is connected to
terminal A.
You can also connect a signal for direction detection to terminal B. If your encoder does not
return a corresponding signal, you can specify the count direction with the user program
using the control interface.
The figure below shows an example of the time profile of the signals of a RS422 pulse
encoder with direction signal and the resulting count pulses:
Count signals of RS422 pulse encoders with Up/Down count signal
The Up count signal is connected to the A terminals. The Down count signal is connected to
the B terminals.
The figure below shows an example of the time profile of the signals of an RS422 pulse
encoder with Up/Down count signal and the resulting count pulses:
Monitoring of encoder signals
The technology module monitors RS422 signals for wire breaks, short-circuits, and offset
voltages.
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