Siemens SIMATIC FM 351 Manual page 123

Signal Evaluation
Increments
An increment identifies a signal period of the two encoder signals A and B. This
value is listed in the specifications of an encoder and/or on its type label.
Signal period= Increment
1
Pulses
Figure 10-2
Pulses
The FM 351 evaluates all 4 edges of the signals A and B (see figure) in each
increment (4x decoding).
1 increment (from encoder)
Reaction Times
With incremental encoders connected, the FM 351 has the following reaction
times:
Reaction time = switching time of the connecting switching elements
Note
You can compensate the minimum reaction time by appropriate parameter
settings for the switchover and switch-off differences.
Unsharpness
Unsharpness affects the positioning accuracy. With incremental encoders the
unsharpness is negligible.
FM 351 Positioning Module
C79000-G7076-C351-02
2 3 4
4x decoding
Increments and Pulses
1 pulse (FM evaluation)
Encoders
A
B
10-3