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.
'Active' and 'passive' modes
A current loop channel has 3 main components; a current source, a switch, and a detector. The component which is the source of the data (the keyboard, in a terminal; the computer, for an output channel, e.g. to the printer of a terminal) will include the switch; the consumer of the data (the printer, in the output channel case) will include the detector.
Either end of the channel can have the current source; the term 'active' is usually used to indicate that end. So an 'active receiver' would be one that includes a current source, and similarly an 'active transmitter'. Both receivers and transmitters can also be 'passive'. Normally, in a bi-directional link, both channels will have their active and passive ends at the same end of the limk, and so the entire interface is described as 'active' or 'passive'.