In building automation systems, multiple entities may control and manipulate an object. There may be a need to arbitrate between a schedule program, manual operation by the facility manager, and from the service technician. For so-called commandable properties (like the present value of output object types), BACnet uses a 16-level prioritization schema where 1 is the highest priority (Manual Life Safety) and 16 is the lowest. The value in the highest priority slot takes precedence over all others. A default value is taken from the property “Relinquish_Default” in case all slots in the priority array are uninitialized (NULL).
Change-of-Value is used to inform client processes about changes of values spontaneously without the need for polling. For analog data a hysteresis (COV-increment) is provided as a special property to allow reducing the traffic and only notify relevant changes.
Note 1 to entry: The mapping to OPC UA concepts is described in Annex B.