Siemens SIMATIC ET 200SP Manual page 50

Assigning parameters / Setting the address space
6.2 Explanation of the parameters
Discrepancy time infinite
If you activate "Discrepancy time infinite", the time period for the discrepancy analysis never
expires. As a result, a time-related discrepancy error will not be detected.
Nevertheless, the following switch-on condition applies: After a channel has been switched
off, a "0" signal must be read in simultaneously at both input channels of the relevant
channel pair before the channel can be switched on again.
If this condition is not satisfied, especially in the event of a single-channel switch-off (i.e.,
signal OFF at only one of the two input channels of the relevant channel pair) followed by a
change to signal ON at both input channels, a discrepancy error will be detected at the time
of the change to signal ON (at both input channels) even though the time for the discrepancy
analysis has not expired. In data record 202, the bit 0.2 "= 1" is set in the status of the input
monitoring of AS-i address n.
A discrepancy error is only detected if both input channels of the affected channel pair have
not received the "0" signal simultaneously.
If "Discrepancy time infinite" is deactivated, the discrepancy analysis uses the value set for
the "Discrepancy time" parameter.
Discrepancy time
You can specify the discrepancy time in the range of 0 to 60 000 ms for each channel pair.
The following settings are required:
● "Sensor evaluation": 1oo2 evaluation, equivalent
● "Discrepancy monitoring": Active
● "Discrepancy time infinite": Deactivated
In most cases, the discrepancy time is started but does not completely elapse, because the
signal differences are evened out again after a short time.
Select a large enough discrepancy time so that in error-free cases, the difference between
the two signals always disappears before the discrepancy time expires.
If the input signals do not match after expiration of the assigned discrepancy time, e.g., due
to a mechanical defect of the sensor or sensor wire break, a discrepancy error is detected.
When discrepancy time is running, the switch-on condition also applies that after a channel
has been switched off, a "0" signal must be read in simultaneously at both input channels of
the relevant channel pair before the channel can be switched on again. If this condition is not
satisfied, especially in the event of brief switch-off of a channel (due to signal OFF at only
one of the two input channels) followed by a change to signal ON at both input channels, a
discrepancy error will be detected at the time of the change to signal ON (at both input
channels) if the time for the discrepancy analysis has not expired.
A detected discrepancy error indicates to the user that the switch-on condition is not
satisfied.
Assign a discrepancy time that is greater than the input delay time.
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Fail-safe Module F‑CM AS‑i Safety ST (3RK7136‑6SC00‑0BC1)
Manual, 03/2017, A5E03957059020A/RS-AB/002