Digital current loop interface

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 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.

Long before the RS-232 standard, current loops were used to send digital data in serial form for teleprinters. Older teleprinters used a 60 mA current loop. Later machines operated on a lower 20 mA current level and most early minicomputers featured a 20 mA current loop interface, with an RS-232 port generally available as a more expensive option.

The maximum resistance for a current loop is limited by the available voltage. Current loop interfaces usually use voltages much higher than those found on an RS-232 interface, and cannot be interconnected with voltage-type inputs without some form of level translator circuit.

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