Fail-Safe Modules
7.5 4/8 F-DI DC24V PROFIsafe Digital Electronic Module
ET 200S Distributed I/O System - Fail-Safe Modules
Installation and Operating Manual, 08/2008, A5E00103686-07
111
Discrepancy Time Parameter
You can define the discrepancy time for each channel pair with this parameter. The entered
value is rounded to a multiple of 10 ms.
Requirements
Parameter settings:
● Sensor evaluation: "1oo2 evaluation"
● Type of sensor interconnection: "2-channel equivalent" or "2-channel non-equivalent"
Discrepancy Analysis and Discrepancy Time
When using a dual-channel, or non-equivalent sensor, or two single-channel sensors which
measure the same physical process variable, the sensors will interact with a slight time delay
because of precision limitations in their arrangement.
The discrepancy analysis for equality/non-equality is used at fail-safe inputs to detect errors
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 match) has disappeared after
expiration of a specified time known as the discrepancy time. If not, this means that a
discrepancy error exists.
In most cases, a discrepancy time is started, but does not fully expire since the signal
differences are cleared within 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.
Response During Discrepancy Time
While the programmed discrepancy time is running internally on the module, either the last
valid valueor "0" is returned to the safety program on the F-CPU by the input channels
involved, depending on the parameter settings for the behavior of discrepancy.
Response During Discrepancy Time
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.
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