What is an HMI
The operator interface of a machine or unit: what the HMI does and does not do, how it talks to the PLC, runtime and engineering, and when a local panel is not enough.
Role
An HMI (Human-Machine Interface) shows the state of the process and lets the operator act: start/stop, change setpoints, acknowledge alarms, select modes, load recipes. The PLC controls; the HMI only requests. If the HMI fails, the process must stay in a safe, controlled state.
Architecture
| Part | Content |
|---|---|
| Engineering software | Screens, tags, alarms, recipes, users — compiled into a runtime project |
| Runtime | Runs on an operator panel or an industrial PC |
| Tags | Links between screen objects and PLC variables (or internal variables) |
| Connection | PLC driver (S7, CIP…), OPC UA or Modbus with a polling cycle |
Acquisition cycle matters: hundreds of tags polled every 100 ms load both the panel and the PLC communication. Poll fast only what is visible on the current screen.
What belongs in the PLC, not the HMI
- Interlocks, sequences, limits and their delays.
- Validation of operator input (the PLC must reject a setpoint outside limits even if the HMI checks it too).
- Anything that must work when the panel is off.
Local HMI vs SCADA
| Local HMI (panel) | SCADA | |
|---|---|---|
| Scope | One machine or unit | Whole plant, several PLCs |
| Data history | Limited, local | Historian, long term |
| Users | Operators at the machine | Control room, engineering, management |
| Redundancy | Rare | Server pairs, redundant networks |
Many plants use both: panels at machines for local operation and SCADA for supervision — see SCADA vs HMI vs DCS.
Connections
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Sources
Updated 2026-10-02