Siemens SIMATIC S7-300 FM 350-1 Manual page 164

Encoder signals and their evaluation
8.3 24-V signals
24-V pulse encoders without/with direction signal
Encoders such as proximity switches (BERO) or light barriers return only a count signal
which you wire to terminal A* of the front connector.
in additional, you can wire a signal for direction detection to terminal B* of the relevant
counter. If your encoder does not return a corresponding signal, you can wire a
corresponding ID signal you generate within the user program, or use a corresponding
process signal.
The diagram shows the time profile of the encoder signals, and the resultant count pulses
Figure 8-3
Programming the encoder inputs
The count direction is defined by programming the encoder inputs. The diagram shows a
change of the count direction based on parameter settings.
Table 8- 2
Programming
current sourcing, push-pull
current sinking
Set the "24 V pulse and direction" parameter for the selected encoder.
You can not reverse the direction of these count signal by inverting the B* signal.
Note
This type of evaluation may cause the count value to "drift off" at the edges if count signal
oscillates, as all signals are added.
164
Signals of a 24-V pulse generator with direction signal
Count direction determined by input parameters
Terminal B*
not wired
24 V connected
not wired
Short-circuited to ground
Count direction
Up
Down
Down
Up
FM 350-1 Counter module
Manual, 05/2011, A5E03648639-01