1.0 – Introduction
Most field devices are passive servers. They wait passively for a system to poll them for data and only then do the devices respond. A consequence of this is that the data client only knows the value of an object property when it polls for the value. If the duration of an event (change of value) is shorter than the interval between polls then the data client will not know the event occurred.
BACnet provides a solution for this by defining services to report events. These services allow a device to be transformed from a passive server to an active server since it is now capable of sending messages reporting the value of an object property based on some event rules. BACnet COV is a sub-set of the ‘Alarm and Event Services’.
This article discusses how the technology operates and then provides a discussion on some weaknesses in the COV system.
2.0 – A simplified definition of BACnet COV
Data clients subscribe to an object for COV reporting. The device monitors the value of the object property, monitors the subscription list and the change criteria. When the change criteria are met the devices sends notifications of the new value to the subscribers.
3.0 – Not all devices / objects support COV
The BACnet COV system is not a mandatory part of the protocol. Each vendor decides if they want to support it. In addition, each vendor gets to decide which properties of which objects support COV.
The device object of a COV server indicates whether there is support for COV. Beyond that you have to look for the presence of certain properties such a “COV Increment’ to tell if an object supports COV or you have to refer to the vendor documentation.
4.0 – How COV Works
Subscribe
Data Client sends messages to the device to subscribe to COV notifications.
The server must accept the subscription. 
Monitor
COV Server device monitors the property values of subscribed objects and the COV change criteria.
Notify
When a change has occurred that meets the COV change criteria the server sends notifications to the subscribers or for those objects with a COV Period defined, notifications are sent based on the period if there has been no change.
Process Notification
The data client must process the notification and update the display, log, etc.
Unsubscribe / Renew Subscription
If the data client no longer needs the subscription it unsubscribe. If it needs the subscription maintained then the client should periodically re-subscribe.
The system is a little more complex than this. For example, subscriptions have durations and there is more than one way the notification can be sent.
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