RSView Supervisory Edition Installation Guide

11 • CREATING ALARMS• • • • •Variable thresholdsThreshold values can be constant or variable. The previous example uses constantthresholds. To define a variable threshold, specify a tag name in the Threshold box whensetting up the thresholds for an analog alarm tag. As the value of the specified tagchanges, the threshold changes.Variable thresholds require more system resources than constant thresholds because of thecontinuous scanning of threshold values, and the processing necessary to detect alarmfaults.11 • PlaceholderAlarm faultsA variable threshold must not become higher than the threshold above it or lower than thethreshold below it. If this happens, an alarm fault is generated for the monitored tag. Tocorrect an alarm fault, change the variable threshold so it does not overlap either of itsneighbors. 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 actions 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 thealarm log file and the printer.• The alarm fault status bit for the tag is set in the value table. This notifies otherapplications that an alarm fault has been generated.• The Alarm Type column in the alarm summary states that the tag is in ‘Alarm Fault’.When the faulty thresholds return to their normal operating range, the alarm faultcondition is cleared, the out-of-alarm-fault status is generated and logged, and alarms forthe tag resume normal operation.DeadbandWith some measured values, such as line pressure, tag values can fluctuate rapidly aboveand below a critical threshold. Where such conditions exist, you can create a deadband asa buffer to prevent the fluctuations from triggering unnecessary alarms.If the threshold is increasing—monitoring rising values—the deadband range lies belowthe threshold. If the threshold is decreasing—monitoring falling values—the deadbandlies above it.11-5