Siemens SIMATIC Operating Instructions Manual page 60

Fail-Safe Electronic Modules
8.1 8 /16 F-DI DC24V PROFIsafe Digital Electronic Module
Short-Circuit Test Parameter
The cyclic short-circuit test is enabled and disabled using the short-circuit test parameter.
The short-circuit test is only useful for simple switches that do not have their own power
supply. If the short-circuit test has been enabled, the internal sensor supplies must be used
(see also
Behavior at Discrepancy Parameter
As the "behavior at discrepancy" you assign the value that is made available to the safety
program in the F-CPU while there is a discrepancy between the two input channels involved,
i.e., during the discrepancy time. You assign the behavior at discrepancy as follows:
● "Provide last valid value", or
● "Provide value 0"
Requirements
You have assigned the following:
● Evaluation of the sensors: "1oo2 evaluation"
"Provide last valid value"
The last valid value (old value) before discrepancy occurs is made available to the safety
program in the F-CPU as soon as a discrepancy is detected between the two relevant input
channel signals. This value is supplied until the discrepancy disappears or until the
discrepancy time expires and a discrepancy error is detected. The sensor-actuator response
time is extended by an amount equal to this time.
As a result, the discrepancy time of sensors connected via two channels must be set for fast
reactions to short response times. It makes no sense, for example, for a time-critical
shutdown to be triggered by sensors connected via two channels with a discrepancy time of
500 ms. In the worst case, the sensor-actuator response time is extended by an amount
approximately equal to the discrepancy time.
● For this reason, position the sensors in the process in such a way as to minimize
● Then select the shortest possible discrepancy time that includes a sufficient cushion
"Provide value 0"
The value "0" is made available to the safety program in the F-CPU as soon as a
discrepancy is detected between the signals of the two relevant input channels.
If you specified "Provide value 0", the sensor-actuator response time will not be affected by
the discrepancy time.
60
"Use Cases of the 8/16 F-DI DC24V PROFIsafe Electronic Module"
discrepancy.
against false tripping of discrepancy errors.
ET 200pro Distributed I/O System - Fail-Safe Modules
Operating Instructions, 05/2007, A5E00394073-02
).