Siemens simatic s7-400 FM 450-1, User Manual, Page: 97 / 130

Encoder signals and their evaluation
9.5 Signal evaluation
Counter module FM 450-1
Manual, 02/2014, A5E03648739-02
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9.4 24-V signals
Count signals returned by 24-V encoders
24-V incremental encoders
The 24-V incremental encoder returns the 24-V signals A*, B* and N* to the module. The A*
and B* signals are phase-shifted by 90°.
24-V signals are marked with an asterisk "*" character.
The tracks A* and B* of a 24-V incremental encoder are used for counting. Track N* is used
to initialize the counter with the load value, if programmed accordingly.
Encoders which do not return inverted signals asymmetrical encoders.
The diagram shows the time profile of the encoder signals:
Figure 9-2 Signals of the 24-V incremental encoder
The module detects the count direction by evaluating the ratio of signals A* and B*. The
diagrams in the chapter "Signal evaluation (Page 100)" show which edges of the A* and B*
signals are incremented or decremented.
You can program the inputs of 24-V encoder signals for the connection of source outputs, or
push-pull outputs, or sink outputs. For further information, refer to the encoder manual.
You can change the count direction using the "Count direction normal" and "Count direction
inverted" parameters without having to modify the wiring.