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

You can use Persistence to prevent momentary changes in point value from changing the state of a point. When used, persistence stops the driver from processing the 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 specify the type of persistence using a point’s Persistence Type field. You 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. The persistence fields are on the <Point Type> tab of the Point Form.

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.

• Away From Normal—This option is only available for analog points. This type of persistence is designed for points on which periodic changes of value constantly leave the point's value outside of the Normal range of values. It is designed to help capture those state changes that persist for long enough outside of the Normal range, rather than erroneously leave the point persisting in the Normal state. It is a form of Into State persistence in which multiple state persistences can be active at one time. The states that are above the Normal range (such as High, High High, and Full Scale) form one group for persistence purposes. The states that are below the Normal range (such a Low, Low Low, and Zero Scale) for another group for persistence purposes.

  In each case, when a point's value travels into one of those groups of states, within that group the persistence timer of the relevant state that is closest to the Normal state prevails. (That is, the High state, or the Low state persistence timer, as applicable.) This timer continues to run even if the point's value continues to travel through that state (or states) into another state that is even further away from the Normal range, providing that the new state is in the same 'group' of states for persistence purposes. The persistence timer of that further state then also starts to run. (So, for example, a value change from a Normal value into a Full Scale value might cause persistence to run against High, High High, and Full Scale.)

  Once there is an expiry of an Away From Normal persistence period:

  • New changes in point value that continue in the same direction away from the Normal state can trigger new persistence periods.

  • Changes in point value that result in the point traveling into a state that is in the direction of the Normal range of values raise a new state change immediately.

  In more detail:

  When a point's value changes, any change of point state is delayed until that 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:

  • Travels back out of that state, into a state that is closer to the Normal state

  • Returns to the Normal state

  • Travels past the Normal state into the other group of states for persistence purposes (for example, into the Low state when it was previously in the High state)

  the persistence timer for the state out of which the point's value has traveled is canceled.

  If, within the Persistence Duration, the point value changes further in the same direction, such that the point enters another state that is even further away from the Normal state, the persistence timer for the last state will continue to run. Additionally, the persistence timer for the new state will start to run. If, within the original Persistence Duration, the point value changes again, such that the point travels back into the state that triggered the original persistence timer to run, the persistence timer for the state into which it fleetingly traveled will be canceled. However, the original Persistence Duration (for the original point state that triggered the initial persistence timer) will still continue to run, and when it expires, will record the point as being in that state. So, for example, if a point's value fluctuates between the High and High High states, only the High High persistence timer will be canceled whenever the point's value drops back into the High state. The persistence timer for the High state will continue to run, providing that the point's value does not drop lower than the lowest limit that is configured for the High state.

  When traveling in a direction that is away from the Normal state, whenever a persistence timer expires for a limit that is further away from the Normal state than another limit, any persistence timers that are running for limit(s) that are closer to the Normal state cease automatically.

  Whenever a point's value travels in a direction that is towards the Normal state, and crosses a limit boundary, no persistence applies in that direction. Any changes to the point value and state are recorded as soon as the point crosses a limit boundary when traveling towards the Normal state.

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

With binary points, a Persistence Duration field exists for each point state.

With analog points, a Persistence Duration field exists for each state limit. (With the DNP3 driver, persistence does not apply when an analog point's value travels into the Invalid state, as that state is 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.)

How Significant Change affects when and if an analog point's change in value and state is recorded

An analog point’s Significant Change settings 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.

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.)