Points – Project Haystack


OverviewContainmentPoint FunctionPoint KindsPoint Min/MaxCur PointsWritable PointsHistorized PointsExamples


Points are typically a digital or analog sensor or actuator (sometimes called hard points). Points can also represent a configuration value such as a setpoint or schedule (sometimes called soft points). Point entities are tagged with the point tag.


All points must be associated with a site via the siteRef tag and a specific piece of equipment via the equipRef tag. Optionally points may define a spaceRef tag if the containing space is defined.

Point Function

All points must define exactly one of the following pointFunction marker tags:

  • sensor: input, AI/BI, sensor
  • cmd: output, AO/BO, actuator, command
  • sp: setpoint, internal control variable, schedule, soft point

Point Kinds

Points must be classified as analog, digital, or multi-state using the kind tag:

  • Bool: model digital points as true/false. Bool points may also define an enum tag for the text to use for the false/true states
  • Number: model analog points such as temperature or pressure. These points should also include the unit to indicate the point's unit of measurement.
  • Str: models an enumerated point with a mode such as "Off, Slow, Fast". Enumeration points should also define an enum tag.

Point Min/Max

Analog points may define a minimum and/or maximum for the point:

When these tags are applied to a sensor point, they model the range of values the sensor can read and report. Values outside of these range might indicate a fault condition in the sensor.

When these tags are applied to a cmd or sp, they model the range of valid user inputs when commanding the point.

Cur Points

The term cur indicates synchronization of a point's current real-time value. By real-time we typically mean freshness within the order of a few seconds. If a point supports a current or live real-time value then it should be tagged with cur marker.

The following tags are used to model the current value and status:

  • curVal: current value of the point as Number, Bool, or Str
  • curStatus: ok, down, fault, disabled, or unknown
  • curErr: error message if curStatus indicated error

Writable Points

Writable points model an output or setpoint, which may be commanded. Writable points are tagged with the writable marker. They are patterned using the BACnet 16-level priority array with a relinquish default which effectively acts as level 17. Writable points which may be commanded over the HTTP API via the pointWrite operation.

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

Historized Points

If a point is historized this means that we have a time-series sampling of the point's value over time. Historized points are sometimes called logged or trended points. Historized points should be tagged with the his marker.

Historized points can have their time-series data read/write over the HTTP API via the hisRead and hisWrite operations.

If a point implements the his tag, then it should also implement these tags:

  • tz: all historized points must define this tag with their timezone name (must match the point's site timezone)
  • hisMode: required tag to indicate whether the point is logged by interval, change-of-value, or as a consumption aggregation
  • hisTotalized: optionally defined to indicate a point is collected as an ongoing accumulated value

The current status of historization is modeled with:

  • hisStatus: ok, down, fault, disabled, pending, syncing, unknown
  • hisErr: error message if hisStatus indicated error


Here are examples for the proper tagging of points:

// temp sensor in an air handler unit
id: @whitehouse.ahu3.dat
dis: "White House AHU-3 DischargeAirTemp"
siteRef: @whitehouse
equipRef: @whitehouse.ahu3
kind: "Number"
unit: "°F"
tz: "New_York"

// fan run command with writable support
id: @a
dis: "AHU Fan Run"
kind: "Bool"
enum: "off,on"
siteRef: @b
equipRef: @c
writeVal: false
writeLevel: 16
writeStatus: "ok"

// zone temp setpoint with historization support
id: @123
dis: "Room 706 Temp Setpoint"
kind: "Number"
unit: "°C"
siteRef: @site-id
spaceRef: @room706
equipRef: @thermostat
tz: "London"
hisStatus: "ok"