The No Change feature detailed in this section applies to analog and counter points and to points (other than Derived Points) that have analog- or counter-type processing.
You can use the fields within the No Change section on an Analog or Counter Point Form to specify whether ClearSCADA should raise an alarm or log an event if a point’s value stops changing.
For instance, you might want ClearSCADA to raise an alarm if a particular point’s value doesn’t change by a specified minimum amount for a defined period of time.
You can use the No Change feature to indirectly detect that an instrument has failed, by detecting that the instrument’s value is no longer changing. (Measurement of real physical processes often includes a small degree of random variation on the signal that is being measured. Some instruments indicate failure by ‘flat-lining’—producing a fixed unchanging signal without this random variation. The No Change feature can be used to check that a random variation in signal is no longer occurring.)
Example:
An incremental sensor is attached to a mechanical float.
A mechanical obstruction causes the instrument to fail.
When the instrument fails, the same reading is reported to ClearSCADA without any variation in value.
The No Change feature is used to detect whether changes in value have stopped occurring and to raise an alarm should this happen.
Once you Understand How the No Change Feature Works, use the fields within the No Change section of the Point Form to Configure the No Change Properties for an Analog or Counter Point.
For further examples that demonstrate how the No Change feature can be used, see Example Configuration.
ATTENTION: With many protocols, in order to use the No Change feature on points that are on PSTN outstations, you need to ensure that ClearSCADA is configured to raise Alarms FromLogged Data Only, or Most Recent Logged Data. With these options the current value reported at the beginning of a call does not inadvertently generate a No Change alarm or event. (For more information, see the ClearSCADA Guide to Advanced Drivers.)
The No Change feature is used to detect whether a point’s value has stopped changing but is still being updated, rather than whether a point has stopped updating.
To check whether a point has stopped updating, look at the ‘Last Updated’ status attribute on the point’s Status display. To ascertain whether more than one point has stopped updating, either display a Points List and sort or filter the List by ‘Last Updated’ time, or create a suitable report.