industrial//industrial data layer//tag

The basic entity of SCADA, historian and OPC systems: a named variable. Tank01.Level, Pump03.Speed, Boiler02.Temperature.

The basic entity of SCADA, historian and OPC systems: a named variable. Tank01.Level, Pump03.Speed, Boiler02.Temperature.

name: Tank01.Level

value: 74.3 %

timestamp: 18:03:41.215

quality: GOOD

An isolated number, 74.3, is worth very little. The name says what it is, the timestamp says when it was true, 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.

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).