Geo SCADA: Understanding the 'No Change Alarm'

The ‘No Change Alarm’ in Geo SCADA is a bit misleading.  One could interpret the functionality as generating an alarm should there be “zero” change to the value after a period of time has elapsed.
This could be construed as a value not updating due to a communication failure; but this is not how this functionality works.

The ‘No Change Alarm’ will not generate an alarm if Geo SCADA loses communications with an outstation, thereby causing the point’s value not to change due to no updates coming in for the point.

The ‘No Change Alarm’ works as follows:



Above is a screen shot showing the ‘No Change’ configuration for an analog point.  With these settings:

• An alarm will be raised if the latest polled value does not change by more than the amount specified in the deadband field.  In this case, the value must change by 5 or more within a 30 second period.
• An alarm is only triggered by comparing the newest value to the previously updated value.
• If the point is updated within 30 seconds with a value that differs by 5 or more from the previous value, then the alarm counter is reset.
• If the point is updated within 30 seconds with a value that differs by less than 5 from the previous value, then when the 30 seconds has expired, an alarm will be generated
• A communications failure will not generate an alarm as there is no new value to compare against the previously updated value

Setting the ‘Type’ field to ‘Percentage of Span’ works in a similar way as follows:





In this example we have a Full Scale value of 200 and a Zero Scale value of 0.
The ‘No Change’ alarm is set to trigger for a change in value of 10% of the span.  Since the span is 0 to 200, a 10% change would be a value of 20.
Therefore, the same rules apply as per the 1^st example, only this time the alarm will trigger if the value does not change by 20 or more.

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