An HMI communicates with a PLC or controller through standard industrial communication protocols that allow data to be shared between both devices. The HMI (Human Machine Interface) acts as the operator’s screen to monitor and control processes, while the PLC (Programmable Logic Controller) executes the logic behind those operations. The communication happens when the HMI sends and receives data to or from the PLC using protocols like Modbus RTU, Ethernet/IP, PROFINET, or RS-232/RS-485 serial connections. These connections help the HMI display live information, such as machine speed, temperature, or alarms, which the PLC continuously updates in real time.
The PLC and HMI must be properly configured so they understand each other’s data addresses. For example, the PLC holds process variables like pressure or motor status in its memory, and the HMI reads or writes to those memory locations to show or change values. Modern HMIs often use Ethernet communication, which is faster and supports multiple PLCs in the same network. The HMI software defines “tags” that link display elements (like buttons, indicators, or meters) to specific PLC data points. When an operator presses a button on the HMI, that command is sent instantly to the PLC, which executes the control logic.
In simple terms, the HMI and PLC work as a team — the PLC controls the process, and the HMI gives the operator a window to see and manage it. This real-time communication ensures better monitoring, faster troubleshooting, and safer operations. Choosing compatible communication protocols and configuring both devices correctly is essential for a smooth and reliable connection between the HMI and PLC.