HMI stands for Human Machine Interface, which is a device that allows a human to give directions and receive feedback from the PLC that is controlling the manufacturing process. In other words, it is a means to input commands into machines and receive feedback about their status. HMIs are used as an operator control panel to PLCs, RTUs, and in some cases directly to IEDs. They are the monitors and controls of a machine that allows the operator to run a machine or monitor a machine.

The HMI and PLC need to be compatible, meaning they need to be able to “talk” to each other. They do this on what is called a Protocol. Different companies use different protocols. Common protocols are Modbus, Ethernet/IP, and Profibus. Once the PLC and the HMI are “talking,” then whatever is programmed into the HMI can be used to monitor and control PLC functions.

The programming that will control the inputs and outputs of an HMI will generally reside on the PLC, giving the PLC programmer most of the control over the functionality of how the HMI will operate. However, both of these functions are combined at most facilities, and the PLC programmer would either create the layouts of the HMI screens or be familiar enough with the process to dictate how HMI programming will take place.

In summary, HMI is a device that allows humans to communicate with the PLC via a graphical interface, typically a touchscreen. It allows operators to monitor and control processes in an automated system.