Siemens SIMATIC S7 Functional Safety Manual page 110

Fail-Safe signal module (SM) I/O configuration
5.3 Configuring SM 1226 F-DI 16 x 24 VDC DI and channel parameters
Note
Safety program access to 1oo2 input data
For 1oo2 evaluation, two paired input channels (for example, F-DI a0.0 and F-DI b0.0) are
connected to one or two sensors. The F-DI performs the discrepancy analysis and sends the
result to the safety program, at the input address of the low-numbered channel (F-DI a0.0, in
the example).
Sensor-actuator response time is increased when you add more input discrepancy time
The discrepancy time adds directly to the maximum response time of a 1oo2 evaluation if the
two signals do not agree in logical state. If you select the "Supply value 0" option, then the F-
DI does not delay a transition from "1" to "0", but can delay the transition from "0" to "1". If
you select the "Supply last valid value" option, then the F-DI can delay both transitions from
"1" to "0' and "0" to "1". The discrepancy time is influenced by sensor specifications,
installation tolerances, and wiring. For best response time, select the smallest discrepancy
time that provides reliable normal operation. Refer to "Fail-Safe signal module (SM)
response times" (Page 197) for further information.
Discrepancy time variation with short circuit test
If your process input changes near a short-circuit test, a discrepancy is detected in less time
than your configured discrepancy time.
With:
Tdisc = configured discrepancy time
Tsct = configured short circuit test duration
Tfilter = configured input filter time
Tda = actual discrepancy time, time between process signal changes that can be detected
as a fault
The range of detected discrepancy time is:
If Tfilter < Tsct: {Tdisc - (Tfilter + Tsct)} <= Tda <= Tdisc
If Tfilter >= Tsct: {Tdisc - (2 x Tsct)} <= Tda <= Tdisc
Your configured discrepancy time should account for this variation to avoid unexpected
passivations.
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S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA