Key Elements of the COV Technology

COV Server :

A BACnet device that supports COV, accepts subscriptions and sends COV notifications messages to a COV Client.

COV Client :

A BACnet device, typically a SCADA or Logging application, which can subscribe for COV notification and which can process the notification messages.

Subscription :

Establishes a relationship between a COV Server and a COV Client.

Subscriptions have the following attributes :

• Subscription to an Object or to a Property BACnet provides two services for subscription. One subscribes to an object and the other to a property of an object.
• Identification of the Subscriber The server needs to know where to send the notifications.
• Object Identifier Eg. Analog Input 1 If subscribing to a property then the property must be identified too.
• Lifetime Is indefinite or a specific number of seconds. Values can be large.
• Notification Type Notification can be sent with/without requiring confirmation from the data client.
• COV Increment This parameter is only used in subscriptions to object properties. If not specified in the subscription the object uses its own increment.

Notification :

A message which reports the current value of the changed property as well as the current state of the objects Status Flag property if it exists. The notification also contains the number of seconds remaining to the subscription. Confirmed notifications require a response from the COV client.

6.0 - Change Criteria

The change criteria are based on the type of subscription

Subscribe to the object :

Either of these cases trigger notification:
1) If the status flags change at all
2) If the Present Value changes:

• Binary, Life Safty and Multistate Objects : any change to Present Value
• Analog, Loop and Pulse Objects: change by COV_Increment

Subscribe to a property :

Either of these cases trigger notification:
1) If the status flags change at all (if the object has status flags)
2) If the Property Value changes:

• Property is of type REAL : change by COV_Increment ( which may be defined in the subscription or may be the native increment defined in the device).
• Property is of some other type: any change to Present Value