What is an HMI?
The HMI is the screen operators use to run a machine: start and stop it, enter setpoints, see what it is doing and react to alarms. It is the face of the PLC.
How an HMI talks to the PLC
An HMI does not control anything by itself. It is connected to the PLC over Ethernet (or serial) and works with tags: each object on the screen is linked to a PLC address.
- Pressing a button writes a bit in the PLC (for example M0.0).
- A lamp or a number on the screen reads a bit or a word from the PLC.
- The PLC program decides what actually happens.
Typical screen objects
| Object | Linked to | Example |
|---|---|---|
| Push button (momentary) | bit, ON while pressed | Jog, Start |
| Set / reset / toggle button | bit | Auto / manual mode |
| Indicator lamp | bit | Pump running |
| Numeric display | word / register | Tank level 72 % |
| Numeric input | word, with min/max limits | Fill set point |
| Bar graph, gauge | word | Pressure |
| Trend | word, logged over time | Oven temperature |
| Alarm banner / list | alarm bits | High level, motor overload |
Alarms, recipes and users
- Alarms: each alarm bit has a text, a priority and a time stamp. Operators acknowledge alarms, and the history shows what happened.
- Recipes: sets of values for each product (speed, temperature, quantities) that the operator selects and downloads to the PLC in one go.
- User management: passwords or badges; operators can run the machine, maintenance can change settings, engineers everything.
- Data logging: production counts and process values saved to a file or database.
HMI software by brand
| Brand | Panels | Software |
|---|---|---|
| Siemens | SIMATIC Comfort / Unified panels | WinCC in TIA Portal |
| Mitsubishi | GOT2000 / GOT3000 | GT Designer3 |
| Rockwell | PanelView Plus | FactoryTalk View ME / Optix |
| Omron | NA / NB series | Sysmac Studio / NB-Designer |
| Weintek, Pro-face | many models, talk to many PLC brands | EasyBuilder Pro, GP-Pro EX |
HMI vs SCADA
An HMI usually belongs to one machine and sits on it. SCADA (Supervisory Control and Data Acquisition) runs on PCs and supervises a whole plant or many sites: many PLCs, large alarm and history databases, reports.
Golden rules
- The HMI writes to memory bits and setpoints, never directly to outputs. The PLC checks interlocks.
- An emergency stop is always a hard-wired button, never an HMI button.
- Momentary HMI buttons can stay ON if the connection drops: the PLC should use them as pulses or with a timeout.
- Show the real state (from sensors and feedback), not just what was commanded.
Next: how to design a good HMI screen. The PLC Sim Lab Playground has an HMI touch panel you can configure and connect to the PLC.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →