A current loop is a communication interface that uses current instead of voltage for signaling. Even if there is significant electrical resistance in the line due to long length or poor connections, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. For industrial process control instruments, 4-20 ma and 10-50 mA current loops are commonly used for analog signaling, with 4 ma representing the lowest end of the range and 20 ma the highest. Current loops can be used over for long-distances and for optically isolated links.

Long before the RS-232 standard, current loops were used to send digital data in serial form for teleprinters. More than two teletypes could be connected on a single circuit allowing a simple form of networking. Older teletypes used a 60 ma current loop. Later machines, such as the ASR33, operated on a lower 20mA current level and most early minicomputers featured a 20 ma current loop interface instead of an RS-232 port. The original IBM PC Serial port card had provisions for a 20 mA current loop. A digital current loop uses the absence of current for high (mark), and the presence of current in the loop for low(space).

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.