Issue
When using COV with BACnet and Niagara R2, what object types should be used with it?
Environment
- BACnet
- Niagara R2
- COV
Cause
To get the most benefit from implementing COV and make certain it performs well.
Resolution
Per the Niagara R2 BACnet Integration Manual follow these COV Strategies:
Best Candidates for COV: In general, the best usage of COV is for objects that have values that change infrequently - typically binary and multistate input objects or binary and multistate value objects. These are represented by BACnet shadow object types BI, BV, MSI, and MSV. You can also use COV for analog input and value objects (AI and AV). However, to reduce notifications you would have to increase each object’s covIncrement value, which would also reduce sensitivity to changes - consider that only changes larger than the covIncrement value will be reported.
Poor Candidates for COV: Starting in release r2.3.4, the priorityArray property of output-type objects (AO, BO, MSO) and value-priority objects (AVP, BVP, MSVP) is polled, in addition to presentValue and statusFlags properties. This is done as part of a “rewrite mechanism” (see “Rewrite Mechanism,” page 3-25). Although you can configure any of these BACnet objects to useCOV, the object remains in the poll queue to receive priorityArray - as this property cannot be received using SubscribeCOV logic. Therefore, configuring these objects to useCOV will not significantly reduce message traffic. The poll message for each will request only one value instead of three, but this typically makes little difference. Therefore, it is recommended that you do not configure any output-type and value-priority objects for COV usage, unless the device can support plenty of COV subscriptions, and you have already subscribed all of the input and “non-priority” value type objects.
