HMI is an abbreviation of the English term Human - Machnie - Interface, meaning the interface or connection between man and machine. These devices are typically the following:
- Keyboards, keypads, touch screens, buttons and other input devices.
- Indicator LEDs, displays, speaker and other output devices.

We call complete units that perform these human-machine interface functions in one, and are connected to the central controller of the equipment using some kind of communication protocol, an HMI unit.
It is advisable to treat HMI units as separate elements, because this way their repair and replacement can be done simply and quickly, unlike if the HMI is completely embedded in the central equipment.