industrial//industrial data layer//tag

A tag is the basic entity of SCADA, historian and OPC systems: a named variable such as Tank01.Level, Pump03.Speed or Boiler02.Temperature.

A tag is the basic entity of SCADA, historian and OPC systems: a named variable such as Tank01.Level, Pump03.Speed or Boiler02.Temperature.

A useful datum is four things: name, value, timestamp and quality.

nameTank01.Level

value74.3 %

timestamp18:03:41.215

qualityGOOD

An isolated number, 74.3, is worth very little. The name says what it is, the timestamp says when it was true (the event time at the source, which a late delivery must not overwrite), and the quality says whether it can be trusted at all: a stale value from a transmitter that lost power is still a number, and quality is what marks it BAD or UNCERTAIN.

A tag is a value sampled over and over, and it is easily confused with an event, something that happened once. Storing a pump state as a tag gives a flat line with steps, storing it as events gives PumpStarted at 18:03:41; both are needed and they are not interchangeable.

Quality is one of the things a control protocol carries and a plain HTTP response does not, which is part of fault semantics. Tags are what a historian stores by the billion and what an edge gateway normalizes when it renames Modbus register 40117 into Boiler01.SteamPressure (data contextualization).