Client COV notes

Niagara offers client support for BACnet COV services—for devices that support COV. Receiving COV (change-of-value) notifications for proxy point data is typically more efficient than polling for data.

To take advantage of this, under the BacnetNetwork you need to add at least one additional TuningPolicy (apart from the DefaultPolicy), with its “Use Cov” property set to true. Then assign Bacnet proxy points under a COV-capable BacnetDevice to such a “CovPolicy”. The driver then attempts to subscribe to that BACnet device for COV notifications.

Figure 19. Example proxy points subscribed for COV

If successful, such a proxy point will list in the Bacnet Point Manager “Read” column showing “COV” instead of “Polled”, as shown above in Figure 19. If the COV subscription is unsuccessful, the point will fall back to polling and show “Polled”, like points configured with a polling TuningPolicy. For related details, see Bacnet-only Tuning Policy properties. Also see the next section, Changes in COV statusFlags reporting.

Changes in COV statusFlags reporting

Until recent releases of the BACnet driver, the default behavior varied between polled Bacnet proxy points and proxy points that use COV, as follows:

Polled points, by default, reflect only the selected BACnet property (typically “Present_Value”), but not any BACnet “Status_Flags” that be set on source objects. Only Niagara point status is seen. Thus, a source BACnet object may be in a native alarm state, but this is not indicated by the proxy point. Any alarm state requires a Niagara alarm extension on the point. If needed, you can change this behavior at the point level. See Facets usage to poll additional properties.
COV points, by default, did reflect any BACnet “Status_Flags” on the source objects, in addition to the selected BACnet property (typically “Present_Value”). Therefore, a Bacnet proxy point might have an alarm status, yet there would be no Niagara alarm extension on the point. A “status merger” was used (sometimes causing confusion). See Status merger for Bacnet proxy points.

Starting in AX-3.7u1, the default COV proxy point statusFlags behavior changed to match polled proxy point behavior. In other words, only Niagara point status is seen by default. If needed, you can change this back to the previous COV proxy point behavior, at the point level, by adding a “statusFlags” facet to points. See Facets usage to poll additional properties.