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PANEL PCS · HMI

How Does an HMI Work With a PLC?

A PLC runs the control logic of a machine; the HMI is the screen operators use to see and change it. The HMI connects to the PLC over an industrial network, usually Ethernet with a protocol such as Modbus TCP, EtherNet/IP, PROFINET or OPC UA, or over RS-485 serial, reading values and writing setpoints and commands back.

01

The Short Answer

The PLC (programmable logic controller) does the work: it reads sensors, runs the program and drives motors and valves. The HMI does no control itself. It reads tags from the PLC, such as temperatures, states and counts, displays them, and writes an operator’s commands and setpoints back to the PLC.

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    Modbus TCP and RTU Simple, widely supported, over Ethernet or RS-485.
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    EtherNet/IP Common in North American automation.
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    PROFINET Common in European automation.
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    OPC UA Vendor-neutral and secure, increasingly used between PLCs, HMIs and SCADA.
02

PLC-HMI Units and PC-Based HMIs

Small machines sometimes use a combined PLC and HMI in one housing. Larger machines usually keep them separate: the PLC in the control cabinet and a panel PC HMI on the door or a swing arm. A panel PC can talk to PLCs from different makers through protocol drivers or OPC UA, and can run other software alongside the HMI.

03

What to Check on the Hardware

Make sure the HMI has the ports the network needs: Gigabit Ethernet, isolated RS-485 for older serial devices and, in wet areas, sealed M12 connectors. Our guide to choosing an industrial HMI covers the full specification.