Current desired value to write to output.
Writable points are points which model an output or setpoint and may be commanded. Writable points are modeled on the BACnet 16-level priority array with a relinquish default which effectively acts as level 17. Writable points which may be commanded by the pointWrite operation should be tagged with the
The following levels have special behavior:
- Level 1: highest priority reserved for emergency overrides
- Level 8: manual override with ability to set timer to expire back to auto
- Default: implicitly acts as level 17 for relinquish default
The priority array provides for contention resolution when many different control applications may be vying for control of a given point. Low level applications like scheduling typically control levels 14, 15, or 16. Then users can override at level 8. But a higher levels like 2 to 7 can be used to trump a user override (for example a demand response energy routine that requires higher priority).
The actual value to write is resolved by starting at level 1 and working down to relinquish default to find the first non-null value. It is possible for all levels to be null, in which case the overall write output is null (which in turn may be auto/null to another system). Anytime a null value is written to a priority level, we say that level has been set to auto or released (this allows the next highest level to take command of the output).
The following tags are used to model the writable state of a point:
writeVal: this is the current "winning" value of the priority array, or if this tag is missing then the winning value is null
writeLevel: number from 1 to 17 indicate the winning priority array level
writeStatus: status of the server's ability to write the last value to the output device: ok, disabled, down, fault.
writeErr: indicates the error message if writeStatus is error condition