What is a tag?
A tag is a unique name or identifier used to designate a specific data element or variable in an automation system. Tags form the core of communication between systems such as PLC, SCADA, HMI, MES and Historian.
They typically refer to sensor data, status information or control parameters within a process.
🧭 What does a tag represent?
A tag may, for example, refer to:
- The temperature of a furnace (e.g.
Oven1.Temp) - The position of a valve (
Valve_A23.Position) -
A Setpoint for rotational speed (
Pump1.RPM_Setpoint) - A fault status (
Compressor1.Fault)
Tags are updated in real time and form the interface between human and machine (HMI) or between machine and database.
📚 Types of tags
| Type | Example |
|---|---|
| Input tag | Value from a sensor (e.g. temperature) |
| Output tag | Control of an actuator (e.g. motor on/off) |
| Setpoint tag | Desired value in a [[Control Loop |
| Alarm tag | Reports when a limit value is exceeded |
| Historian tag | Stored for trend analysis |
🏭 Where are tags used?
Tags appear in, among others:
- SCADA systems: For visualisation, logging and alarming
- HMIs: For operation and visualisation
- MES or Historian: For quality recording and reporting
- PLCs: Internally for logic and control
SCADA systems can manage thousands to millions of tags.
🔗 Tags & standards
In protocols such as Modbus, OPC UA, ProfiNET or MQTT, tags are used to identify data in a structured way. For example, in OPC UA:
ns=2;s=Factory.Pump1.Speed```
📌 In summary
A tag is a digital name linked to a measurement or control value in an industrial installation. Tags form the backbone of data acquisition, control and visualisation in automation systems.