BACnet trigger event - COV

BACnet supports Change of Value (COV) notifications. Even though some devices are not capable of responding to the COV natively, our device driver will pool the device internally and provide the illusion that COV is available from the told device.

Every trigger identifies the trigger event type, which identifies when the trigger will be executed.

The trigger event types are described in the Trigger event type reference. In addition to the standard trigger event types, the BACnet driver makes available additional trigger event type - COV. This trigger event is under within the BACnet event group called COV Change of Value) will subscribe to BACnet COV notifications and raise an event in deviceWISE when a COV occurs. The presentValue property is the only property within a BACnet object that is supported for COV notifications.

The BACnet category is available in the Trigger Event Type list when defining or editing a trigger. This example shows a new trigger being defined with the Canvas Editor:

When the COV trigger event type is selected, the Event tab becomes active with parameters that accommodate the COV event. 

Parameter Description
Filter You will be able to filter COV from a Specific Device or Any Device.
Device name

Device name field appears when Specific Device is selected as a Filter, you should select a device.
Channel Id Channel Id field appears when Any Device is selected as a Filter, you should select a Channel.
Items to subscribe Data in a BACnet device is organized as a series of objects. With this option you will be able to select a type of object to subscribe. You will be able to select from three object types: Analog, Binary, and Multistate.
BACnet Service Type Select Read Property, Read Property Multiple or COV Notifications.
  • If Read Propertyor Read Property Multiple is selected then the presentValue property is polled and events are raised, when the value changes state.
  • If COV Notifications is selected, then a BACnet subscribeCOV will be sent and events are raised on the receipt of a BACnet COV message. In the case where a subscribeCOV notification fails, or if the device does not support COV notifications, the driver will fall back to polling with Read Propertyor Read Property Multiple.
Subscribe COV resubscription interval Enter the interval in which the COV will subscribed.
Specific Items Filter

The "Specific Items" is a comma separated list of objects used to filter the items that will be subscribed. Each item within the list includes the DeviceId, ObjectType and Instance Number in the format:

  • [DEVICE_ID]:[OBJECT_ID]:[INSTANCE]

    • DEVICE_ID is an integer between 0 and 4194302 or * for all devices;

    • OBJECT_ID is an integer between 0 and 512 or * for all object types where 0 is Analog Input, 1 is Analog Output, 2 is Analog Value, 3 is Binary Input, 4 is Binary Output, and 5 is Binary Value;

    • INSTANCE is an integer between 0 and 4194302, * for all instances or a string to match the object name text.

    The text matching the name text can include the "%" symbol at the start or end of the string to ignore the characters before or after the string within the object name. The string can be also wrapped in single quotation marks.

In the case where a device does not support COV notifications, or if the subscription fails, the trigger will fallback to polling for values. On a large system, the number of points can overload the network if all values are subscribed.

It is recommended that the Specific Items Filter be used to limit the subscriptions to points of interest.