An HMI holds the operator interface, while the PLC is the controller that runs the machine’s logic. The HMI displays data and accepts operator input; the PLC executes the programmed instructions that drive the equipment from which data is measured.
The HMI reads from and writes to the PLC over the control network. PLC integration determines how the HMI behaves according to a series of steps:
- HMI tags map directly to PLC memory addresses, so a button press on the screen sets a bit the PLC acts on.
- The PLC runs the deterministic control logic, scanning inputs and setting outputs in milliseconds, while the HMI handles visualization.
- Communication uses an industrial protocol such as EtherNet/IP or Modbus TCP, the messaging standards that carry data in a live feed.
- If the HMI loses communication with the PLC, then the controller keeps running the process while the screen shows a comms-fault alarm.
- Separation of roles means an HMI swap does not require rewriting PLC logic, which protects your validated control program.
The HMI shows and accepts the data, while the PLC runs the logic that actually controls the machine. Explore the controller side of the system on our PLC Panels page.
