HMI (Human-Machine Interface) and PLC (Programmable Logic Controller) are often used together in industrial automation systems, but they are not the same thing.

A PLC is a specialized computing system used to control machinery and processes in manufacturing and industrial settings. It's essentially a ruggedized computer optimized for real-time control tasks. PLCs are programmed to perform specific tasks based on inputs from sensors and other devices, executing a set of logic instructions to produce outputs that control machinery or processes.

On the other hand, an HMI is a graphical interface that allows humans to interact with the PLC and the machinery it controls. HMIs typically consist of a touchscreen or monitor where operators can view real-time data, monitor processes, and input commands. They provide a user-friendly way for operators to monitor and control industrial equipment.

So, while HMIs are often used in conjunction with PLCs to provide a user interface for controlling and monitoring industrial processes, they are separate components with distinct functions.