Siemens SIMATIC ET 200S Installation And Operating Manual page 113

Discrepancy Time Parameter
Here, you can specify the discrepancy time for each pair of channels. The input value is
rounded to a multiple of 10 ms.
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"
Discrepancy Analysis and Discrepancy Time
If you use a 2-channel, one non-equivalent, or two 1-channel sensors, that measure the
same physical process variable, the sensors will respond with a slight time delay among
each other due to the limited accuracy of the arrangement.
The discrepancy analysis for equality/non-equality is used with fail-safe inputs to detect
errors based on the timing of two signals with the same functionality. Discrepancy analysis
starts when different levels (when checking for non-equality: the same level) of two related
input signals are detected. The signals are checked to establish whether the difference
(when checking for non-quality: the match) has disappeared following a selectable time
known as the discrepancy time. If not, a discrepancy error is recognized.
The most cases, the discrepancy time is started but is not elapse fully since the signal
differences disappear after a short time.
Select the discrepancy time large enough so that in case of no error the difference of the two
signals (when checking for non-equivalence: signals matching) has disappeared in every
case before the discrepancy time is elapsed.
Response While the Discrepancy Time Is Running
While the discrepancy time is running internally on the module, either the last valid value or
"0" is provided for the safety program on the F-CPU by the input channels involved
depending on the parameter settings for the behavior of discrepancy.
Response after the Discrepancy Time Elapses
If there is no match of the input signals after elapse of the specified discrepancy time (when
checking for non-equivalence: inequality), for example due to a break in the sensor wire, a
discrepancy error is detected and the "discrepancy" diagnostic message is generated in the
diagnostic buffer of the F-I/O module indicating the faulty channels.
ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04
7.5 4/8 F-DI VDC24V PROFIsafe Digital Electronic Module
Fail-Safe Modules
7-53