For any writable point, the effective input value is determined by a priority scan, looking for a “non-auto” action at level 1 (emergency), then the value at the highest valid input, going from level 2, to 3, and so on to level 16. (At level 8, any “non-auto” action is evaluated as valid).

Like almost all control execution, this priority scan is event-driven, meaning it occurs when any input value changes. An input’s value typically comes from a link—however, note that for most inputs, you may enter a value directly in the point’s property sheet (as an alternative source).

A valid input is one with none of the following status bits set:

If all 16 priority levels are evaluated without a valid input (and without an action at levels 1 and 8), then the fallback value is used.

You can configure the writable point’s “Fallback” property to be “null,” so that the point’s Out has a null status in this condition. Depending on the specific control sequence and usage of the writable point, this may be an effective solution.However, note that by default, a “set” action exists on any writable point, which writes directly to the Fallback value. See About set (Fallback) action. If you want a writable point to always have a Fallback of “null,” go to its slot sheet and set the “Hidden” config flag on the “set” slot. Otherwise, a user can invoke a right-click command to set Fallback to any value. For more details, see Modifying default actions.

Both rules vary from the r2 Niagara priority input scheme, where a single priorityArray input was used for a writable object (AnalogOutput, BinaryOutput, and so forth). That input could be linked to multiple priority type outputs, including those with duplicate priority levels and/or levels also used for object commands (emergency and manual).

The 16 priority levels used by writable points are modeled after corresponding BACnet priority levels, using the following conventions, from highest to lowest:

Although priority levels are patterned after BACnet, there is no direct linkage to BACnet priorities, even with BACnet writable proxy points. Priority inputs of all AX writable points are strictly a station-centric NiagaraAX implementation.