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5-13GFK-0787B Chapter 5 Diagnostics
Analog Input Discrepancy Reporting
If there is a discrepancy in the data from a set of inputs, so that a channel deviates by more
than a configurable percentage from the voted value, the PLC automatically places a message
in the I/O Fault Table where it is available to the Logicmaster 90 software and the application
program logic.
Discrepancy is calculated for engineering units values inputs. Two distinct discrepancy bands
are provided: threshold and limit.
The threshold discrepancy occurs where an A, B, or C engineering units input value
exceeds a specified percentage of the voted value. For example, if channels A, B, and
C report 91, 100, and 111, respectively, the GMR software selects 100 as the
intermediate value. If the threshold discrepancy for the input is set to 10%, this
yields 90 and 110 as the upper and lower threshold discrepancy values. In this
example, channel A is within the threshold band, but channel C is outside, and is
discrepant.
The limit discrepancy occurs where an engineering units input exceeds a given
percentage of the full-scale deflection of the input. For example, if channels A, B, and
C report 9, 10, and 15, respectively, then the GMR software selects 10 as the
intermediate value. If the limit discrepancy is set to 10% of a 200 full-scale deflection
(20 in this case) then no limit discrepancy is reported.
An analog discrepancy is reported where the limit discrepancy and the threshold are both
exceeded. Up to two of the three analog inputs may be discrepant at any given time.
Discrepancy faults are latched, but can be cleared by performing an I/O Fault Reset (see
chapter 7, Programming).
When a discrepancy occurs, the PLC sets the fault contact for that voted input and adapts
according to its configuration. See page 5-25 for information about these fault contacts.
Discrepancy signals are filtered for the configured input discepancy filter time to eliminate
transient discrepancies caused by timing differences.