This is my proposal for tags which are used to express the current real-time value of a point. This is based on the design I developed this summer for SkySpark, which seems to be working out quite well:

• cur: Marker tag to indicate a point has a current value
• curVal: the value of a point or omitted if unknown
• curStatus: Str status as one of the predefined strings:
  • ok: all is okay
  • fault: a configuration problem
  • down: a communication or network problem
  • disabled: manual disable of the point or parent connector
  • unknown: we don't know anything (usually boot state)
• curErr: error message associated when curStatus indicates an error condition

Then use a different set of tags for writable points which are commanding an actuator or setpoint (base on BACnet 16-level priority array):

• writable: annotates a point as a writable output with 16-level priority array.
• writeErr: error message associated with writeStatus
• writeVal: current desired value to write to output
• writeLevel: current priority level for writeVal as number between 1 and 17. The value 17 indicates the relinquish default value.
• writeStatus: string status of a writable point as one of the following predefined strings:
  • unbound: writable point is not bound to an actual I/O point
  • ok: last write was successful
  • down: connectivity/networking problem - see writeErr
  • fault: configuration error - see writeErr
  • disabled: manual disable of the point
  • unknown: we don't know anything (typically boot state)

Does everyone like that or any alternative proposals?

I'm thinking of breaking points out into their own chapter to capture all the different facets and tags used to describe the points.