A COV System (Change of Value) allows devices to warn a Driver if there is a change to a specific property.

To use the COV system, users must enable grouped readings. To do so, configure the Disable Read Grouping and COV as False.

After enabling this system, this Driver starts communicating with a device and checks whether that device supports COV system. If not, this Driver uses grouped or simple readings.

The properties that support COV are presentValue and statusFlags for objects analogInput, analogOutput, analogValue, binaryInput, binaryOutput, binaryValue, lifeSafetyPoint, lifeSafetyZone, multistateInput, multistateOutput, and multistateValue and properties presentValue, statusFlags, controlledVariableValue, and setpoint for the loop object.

To use COV subscriptions, it is recommended that this Driver listens to a fixed port, which is used by a device to send notifications. If the listening port is variable, such as when connecting to a fixed port without specifying a local port, each execution creates new subscriptions, and therefore this Driver cannot remove old subscriptions still active.

Before creating a COV subscription, this Driver checks the existence of a property and then establishes an initial value for the Tag, because COV subscriptions only send values when a property changes.

Notice that in UDP protocol COV notifications sent by a device may be lost, possibly leaving Tags with frozen values. When identifying frozen values, a possible cause could be discarded UDP packets by the network. In this case, disable the COV subscription and return to reading by polling.