For serial communications, a current loop is a communication interface that uses current instead of voltage for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with optically isolated links.
There are a variety of such systems, but one based on a 20 mA current level was used by the Teletype Model 33 and was particularly common on minicomputers and early microcomputer which used these as computer terminals. As a result, most computer terminals also supported this standard into the 1980s.