Use Persistence to Stop Fleeting Changes in Value from Changing a Point State

This topic applies to analog and digital points on the OPC UA driver.

Persistence enables you to stop momentary changes in point value from changing the state of a point. When used, persistence stops the driver from processing fleeting changes in point state and reporting those fleeting changes to the server. This stops Geo SCADA Expert from raising change of state alarms and logging change of state events when a point value is fluctuating.

You can configure persistence for changes into a state, or out of a state, using a point’s Persistence Type field. Use this field in conjunction with the Persistence fields, which define the required amount of time before a change in point value is recorded as a change in point state.

The Alarm Limits section on the <Point Type> tab of an Analog Point and a Digital Point contains the Persistence Type field. You can configure persistence for changes into a state, or out of a state, using a point’s Persistence Type field. Use this field in conjunction with the Persistence duration fields, which define the required amount of time before a change in point value is recorded as a change in point state.

Use the Persistence Type field to specify one of the following:

• None—Persistence is not used for the point. Any changes in point state will be recorded as soon as they occur.
• Into State—When a point’s value changes, any change of point state is delayed until the point has remained in that new state for the specified Persistence duration. Only then is the new state recorded.

  If, within the Persistence duration, the point value changes such that the point reverts back to its previous state, the fleeting change in state will not be recorded and the point will be deemed not to have changed.

  If, within the Persistence duration, the point value changes further, such that the point enters another state, the persistence timer will be restarted. The timer will then run for the Persistence duration of the newly entered state, before recording the point as being in that new state (providing that the point does not enter yet another state before the Persistence duration has elapsed).

• Out of State—When a point’s value changes, any change from its current state has to persist for the specified Persistence duration for that change in state to be recorded. The point can enter any number of other states during this time, as long as it remains out of its original state.

  If, within the Persistence duration, the point value changes such that the point does revert back to its original state, the fleeting change in state will not be recorded and the point will be deemed not to have changed.

With a Persistence Type other than ‘None’, use the point’s Persistence fields to specify the amount of time during which a change in state has to persist, before that change in state is recorded:

• With digital points, a Persistence duration field exists for each point state.
• With analog points, a Persistence duration field exists for each state limit. (Persistence does not apply should an analog point become over range (above Full Scale), under range (below Zero Scale), as these states are reported immediately.)

For each change in state that is to persist before it is recorded, use the relevant Persistence duration field to specify the required time period. Enter the required interval in the OPC Time Format. You can enter the value directly in the field, or use the Interval window (accessed via the field's browse button) to specify the required value. (Leave a Persistence duration field at the default of 0 for any state changes that are to be recorded immediately.)

Be aware that an analog point’s Significant Change settings also have an effect on when and if that point’s change in value and state is recorded.

With a Persistence Type other than ‘None’, whenever an analog point’s value crosses a state limit, the relevant Persistence duration begins.

If the point’s Significant Change Deadband has not been exceeded due to the value change, then the value of the point will only be updated once the Persistence duration expires.

However, if the point’s Significant Change Deadband has been exceeded due to the value change, then the value change will be recorded immediately, but the point state will remain the same. Once the Persistence duration expires, the point’s current value will be logged, along with the new state. This is because persistence applies to the state change and not the value change. (If, during the Persistence duration, the point’s value crosses the state limit back to the original state, the point value will not update and the persistence time will be canceled.)