Current loop
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A current loop describes two different electrical signalling schemes.
[edit] 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 20 mA 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 (space or break), and the presence of current in the loop for low (mark).
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.
MIDI (Musical Instrument Digital Interface) is a digital current loop interface.
[edit] Analog
Analog current loops are used for any purpose where a device needs to be either monitored or controlled remotely over a pair of conductors. The devices can use an entire range of currents, (4-20 mA), or only specific currents, (+6 mA, -14 mA). Only one current level can be present at any time. Devices fall into two categories: control and measurement. Some devices, such as two-way radio remote control consoles, can reverse the polarity of currents and can multiplex audio onto a DC current.
