Fail-Safe Modules
7.6 EM 4 F-DI/3 F-DO DC24V PROFIsafe digital electronic module
ET 200S Distributed I/O System - Fail-Safe Modules
Installation and Operating Manual, 08/2008, A5E00103686-07
143
The short-circuit test is only useful when operating with simple switches which are not
connected to their own power supply.
Short-circuit detection temporarily cuts off the sensor supply. The cut-off period equals the
input delay (= 3 ms) (see also
"Applications for the 4 F-DI/3 F-DO DC24V PROFIsafe
electronic module"
).
Sensor Supply Parameter
This parameter can be used to activate the "internal sensor supply" of the F-module. This
setting is a prerequisite for using the short-circuit test.
Note
When there are different sensor supply parameter settings (internal/external) for the
individual channel groups, the applications shown in the next chapter apply to specific
channel groups.
Discrepancy behavior parameter
For "Behavior of Discrepancy" you assign the value that is to be made available to the safety
program in the F-CPU during the time that a discrepancy exists between two input channels,
i.e., when the discrepancy time is running. To program discrepancy behavior:
● "Provide last valid value" or
● "Provide 0 value"
"Provide last valid value"
The last valid value (old value) from before the discrepancy occurred is immediately made
available to the safety program in the fail-safe CPU as soon as a discrepancy is detected
between the signals of the two input channels involved. This value remains available until the
discrepancy is cleared, or until the discrepancy time has expired and a discrepancy error is
detected. The sensor-actuator response time is extended by this time.
As a result, the discrepancy time for sensors connected over two channels for high-speed
reactions must be tuned to short response times. Thus, it makes no sense, for example, if
sensors connected via 2 channels with a discrepancy time of 500 ms trigger a time-critical
shutdown. In the worst case scenario 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
discrepancy.
● Then select the shortest possible discrepancy time which is also sufficient to compensate
for faulty triggering of discrepancy errors.
"Provide 0 value"
The "0" value is immediately made available to the safety program in the F-CPU as soon as
discrepancy is detected between the signals of the two input channels involved.
If the "Provide 0 value" parameter is set, the sensor-actuator response time will not be
influenced by the discrepancy time.
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