Discrepancy Analysis At The Fail-Safe Digital Input Modules - Siemens S7-300 Installation And Operating Manual

Digital modules

8.2 Discrepancy analysis at the fail-safe digital input modules

8.2 Discrepancy analysis at the fail-safe digital input modules
Discrepancy analyses
There are two types of discrepancy analysis for fail-safe input modules:
● With 1oo2 evaluation in the digital input module
● With redundant modules
Discrepancy analysis with 1oo2 evaluation in the digital input module
The discrepancy analysis is performed in safety mode between the two input signals of the
1oo2 evaluation in the fail-safe input module.
The input signal which is returned to the F-CPU is set to"0" if the input signals do not match
(due to wire break at a sensor, for example) on expiration of the assigned discrepancy time.
A "discrepancy error" diagnostic message which identifies the the faulty channel is
generated in the diagnostic buffer of the module.
Note
The input signals from the process are considered valid within the discrepancy time even
regardless of any difference in the two readings of the redundant input signals.
Value output to the F-CPU within the internal discrepancy time of the module:
 for SM 326; DI 8 x NAMUR: the last valid value (old value) of the affected input channel
 for SM 326; DI 24 x DC 24V: the last valid value (old value) of the affected input channel
For example, if the sensor signal is used to control a filling operation and "0 value" is output,
filling will be stopped by the first of the two discrepant signals after the "0" signal is read. If
the second signal does not return a "0" reading, an error is generated on expiration of the
discrepancy time. Select the last valid value for this example.
"Provide last valid value"
The last value (old value) which was valid before discrepancy is detected is made available
to the safety program in the fail-safe CPU as soon as a discrepancy is detected between the
signals of the corresponding input channels. This value remains available until discrepancy
is cleared or until the discrepancy time expires and a discrepancy error is detected. The
sensor-actuator response time is extended according to this time.
That is, the discrepancy time of a high-speed two-channel sensor must be tuned to short
response times.
It does not make sense to use the signals of two-channel sensors with a discrepancy time of
500 ms to initiate a time-critical shutdown. The worst-case sensor-actuator response time is
extended by an amount approximately equal to the discrepancy time:
● You should for this reason integrate the sensor in the process with minimum discrepancy.
● Select the shortest possible discrepancy time in order to provide sufficient reserve
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or "0" value can be assigned ("discrepancy behavior" parameter)
against incorrect triggering of discrepancy errors.
Installation and Operating Manual, 01/2010, A5E00085586-10
Fail-safe signal modules