Current loop
A current loop describes two different electrical signalling schemes.
Digital
For digital 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.
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 teleprinter, 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.
Analog
For industrial process control instruments, analog 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. 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. The live-zero represented by 4 mA allows the receiving instrument to detect failure of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground(earth), analog isolators may be required when connecting several devices in series.