Siemens SIMATIC PCS 7 Operating Manual page 53

Discrepancy behavior
For "discrepancy behavior", parameterize the value provided to the safety program in the F
CPU during the discrepancy between the two affected input channels – i.e. during running
discrepancy time. You parameterize the discrepancy behavior as follows.
● "Provide last valid value" or
● "provide 0 value"
Discrepancy time
Where "1oo1 evaluation" is concerned, the value displayed is not relevant.
The discrepancy analysis for equivalence/non-equivalence is used for fail-safe inputs in
order to detect errors from the temporal characteristic of two signals with identical
functionality.
The discrepancy analysis is started whenever different levels (when testing for non-
equivalence: the same level) are detected on two associated input signals. A test is run to
see whether, once a configurable period of time known as the discrepancy time has elapsed,
the difference (when testing for non-equivalence: the match) disappears. If not, there is a
discrepancy error. The discrepancy time extends the system response time. Therefore,
select as low a discrepancy arrangement of the sensor as possible in the process.
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Configuring S7F/FH hardware
6.4 Setting system parameters for F-signal modules
53