20mA current loop serial line interface
The 20mA current loop serial line interface was an early standard (de facto, not formal) electrical system for asynchronous serial lines between terminals and computers. It was made popular by the Model 33 Teletype.
Current loop interface, as the name implies, used the presence or absence of a current to encode signals, rather than voltage (as used in the EIA RS-232 serial line interface). The first current loop interfaces used a 60mA current, but this was later reduced to 20mA.
The absence of a current signals space (or high), and the presence of a current is used for mark (low). Thus, in the idle (mark) state (the normal for most lines, most of the time), no power is consumed.
Its main advantages are that it can be used over long distances, and in such applications, line losses are not problematic. Its main disadvantage is that high speeds cannot be used over long distances, as they can with voltage-based systems like RS-232.