Siemens SIMATIC Operating Instructions Manual page 112

Fail-Safe Electronic Modules
8.3 F -Switch PROFIsafe Digital Electronic Module
Discrepancy Time Parameter
Here, you can specify the discrepancy time for each channel pair. The entered value is
rounded to a multiple of 10 ms.
Requirements
You have assigned the following:
● Type of sensor interconnection: "2-channel equivalent" or "2-channel nonequivalent"
Discrepancy Analysis and Discrepancy Time
If you are using one two-channel sensor, one nonequivalent sensor or two single-channel
sensors that are measuring the same physical process variable, the sensors will respond
with a time delay due to the limited accuracy of their arrangement.
The discrepancy analysis for equivalence/nonequivalence is used with fail-safe inputs to
detect faults based on the timing of two signals with the same functionality. Discrepancy
analysis is initiated when different levels (when testing for nonequivalence: same voltage
levels) are detected at two associated input signals. A test is conducted to determine
whether the difference in levels (when testing for nonequivalence: the consistency) has
disappeared within a programmable period known as the discrepancy time. If not, a
discrepancy error exists.
In most cases, the discrepancy time starts but does not elapse completely, since the signal
differences disappear after a short time.
Select a discrepancy time of sufficient length so that in case of no error, the difference
between the two signals (when checking for nonequivalence: the consistency) has definitely
disappeared before the discrepancy time expires.
Behavior While Discrepancy Time is Running
While the assigned discrepancy time is running internally on the module, either the last valid
value or "0" is made available to the safety program in the F-CPU by the relevant input
channels, depending on the parameter assignment for the behavior at discrepancy.
Behavior After Discrepancy Time Elapses
If the input signals are not equivalent following expiration of the specified discrepancy time
(when checking for nonequivalence: no inequality), for example due to wire break at a
sensor line, the system detects a discrepancy error and generates a "discrepancy"
diagnostic message in the diagnostic buffer of the F-I/O module to identify the faulty
channels.
112
ET 200pro Distributed I/O System - Fail-Safe Modules
Operating Instructions, 05/2007, A5E00394073-02