Siemens SIMATIC ET 200S Installation And Operating Manual page 112

Fail-Safe Modules
7.5 4/8 F-DI VDC24V PROFIsafe Digital Electronic Module
Short-Circuit Test Parameter
The cyclic short-circuit test is activated or deactivated 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 activated, the internal sensor power supplies must
be used (see also
Behavior of discrepancy Parameter
As the "Behavior of discrepancy", you set 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;
in other words, during the discrepancy time. You set the behavior of discrepancy as follows:
• "Provide last valid value" or
• "Provide 0 Value"
Requirements
You have set the following:
• Evaluation of the Sensors: "1oo2 evaluation" and
• Type of sensor interconnection: "2-channel equivalent" or "2-channel non-equivalent"
"Provide last valid value"
The last valid value (old value) before discrepancy occurs is made available in the safety
program in the fail-safe CPU as soon as a discrepancy is detected between the signals of
the affected input channels. This value remains available until the discrepancy disappears or
until the discrepancy time expires and a discrepancy error is detected. The sensor-actuator
response time is extended according to the this time.
As a result, the discrepancy time of sensors connected over two channels must be set for
fast responses to short response times. It makes no sense, for example, if sensors
connected over two channels with a discrepancy time of 500 ms trigger a time-critical
shutdown. 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 sensor in the process in such a way as to minimize
• Then select the shortest possible discrepancy time that also has sufficient back-up
"Provide 0 Value"
The value "0" is immediately made available to the safety program in the F-CPU as soon as
a discrepancy is detected between the signals of the two affected input channels.
If you assigned the parameter "Provide 0 value" , the sensor-actuator response time will not
be affected by the discrepancy time.
7-52
"Use Cases of the 4/8 F-DI DC24V PROFIsafe Electronic Module"
discrepancy.
against false tripping of discrepancy errors.
ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04
).