Use COV for fast-changing points on controllers that support it well, and polling for everything else. Mix both and watch the trunk load.
Two Ways to Get a Value
Polling asks a device for a property on a schedule. Change of value (COV) asks the device to notify subscribers when the present value changes by more than an increment, or when a binary state flips. Both return the same objects and properties.
What COV Saves
On a shared MS/TP trunk every poll costs airtime. A subscription sends data only when something changes, so a hundred slowly moving temperatures create far less traffic than polling them every ten seconds. For analog points the COV increment decides how often notifications fire.
Where COV Falls Short
Devices limit how many subscriptions they hold, subscriptions expire and must be renewed, and some controllers implement COV poorly or not at all. A subscription that silently lapses looks like a steady value, which is the worst kind of failure. Always keep a slow poll as a safety net.
Choosing per Point
Subscribe to points that change often and matter quickly, such as alarms and statuses. Poll setpoints and slowly moving values at a relaxed interval. On BACnet/IP the traffic cost is lower, so polling more is acceptable.
Sizing the Trunk
Estimate requests per second per trunk from the poll intervals and the expected notification rate, then stay well below what the baud rate allows. If a trunk is saturated, lengthen intervals or raise COV increments before adding another reader.
Common Questions
What is a COV increment?
The amount an analog present value must change before the device sends a notification. A larger increment means fewer notifications and coarser history.
Do all BACnet devices support COV?
No. Support and subscription limits vary by device, so check each controller and keep a slow poll as a safety net.