The 20 mA interface is a relatively old interface that was and is used for connecting peripheral devices such as printers or scanners.
The 20 mA interface is also known as a current loop interface. It works with two pairs of wires on which the transmit and receive signals are transmitted. The information is in the current, unlike other interfaces that work with voltages. The transmission is done in on-off keying, where the current either flows or does not flow. In order to achieve a potential-free connection of the connected devices, an optocoupler can be integrated in the interfaces, which electrically isolates the devices from each other.