Rockwell Automation FactoryTalk View Site Edition Users Guide User Manual
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Variable thresholds
Threshold values can be constant or variable. The previous example uses constant
thresholds.
To define a variable threshold, specify a tag name when setting up alarm thresholds for an
analog tag. As the value of the specified tag changes, the threshold changes.
Alarm faults
A variable threshold must not become higher than the threshold above it or lower than the
threshold below it. If this happens, an alarm fault is generated for the tag being monitored.
To correct an alarm fault, change the variable threshold so it does not overlap either of its
neighbors. This can be complex when the neighboring thresholds are also variable,
because these boundaries are determined dynamically at run time.
When an alarm fault is generated, the following events occur:
The tag’s alarm status stays as it was before the fault was generated.
An alarm fault is reported to all alarm reporting devices you have set up, such as the
alarm log file and the printer.
The alarm fault status bit for the tag is set in the value table. This notifies other
applications that an alarm fault has been generated.
Alarm Fault is posted in the Alarm Type column in the alarm summary.
When the faulty thresholds return to their normal operating range, the alarm fault
condition is cleared, the out-of-alarm-fault status is generated and logged, and alarm
monitoring for the tag returns to normal.
Deadband
With some measured values, such as line pressure, tag values can fluctuate rapidly above
and below a critical threshold. Where such conditions exist, you can create a deadband as
a buffer to prevent the fluctuations from triggering unnecessary alarms.
If the threshold is increasing (monitoring rising values), the deadband range lies below the
threshold. If the threshold is decreasing (monitoring falling values), the deadband lies
above it.
Variable thresholds use more system resources than constant thresholds, due to the continuous
scanning of threshold values, and the processing necessary to detect alarm faults.