# Digital current loop interface

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{{Short description|Serial communications scheme}}
{{about|Digital serial communications|other uses|Current loop}}
thumb|right|RS-232 / Current loop converter
For [serial communications](/source/serial_communications), a '''current loop''' is a communication interface that uses [current](/source/electrical_current) instead of [voltage](/source/voltage) for signaling. Current loops can be used over moderately long distances (tens of kilometres), and can be interfaced with [optically isolated](/source/opto-isolator) links.

There are a variety of such systems, but one based on a 20 mA current level was used by the [Teletype Model 33](/source/Teletype_Model_33) and was particularly common on [minicomputer](/source/minicomputer)s and early [microcomputer](/source/microcomputer) which used these as [computer terminal](/source/computer_terminal)s. As a result, most [computer terminal](/source/computer_terminal)s also supported this standard into the 1980s.

==History==
Long before [RS-232](/source/RS-232) was made a recommended standard in 1960, current loops were used to send digital data in [serial](/source/Asynchronous_start-stop) form for [teleprinter](/source/teleprinter)s.  More than two teleprinters could be connected on a single circuit allowing a simple form of networking.<ref name=PARR98 >E. A. Parr, ''Industrial Control Handbook'', Industrial Press Inc., 1998, {{ISBN|0831130857}}, page 648</ref> Older teleprinters used a 60 [mA](/source/Ampere) current loop.  Later machines, such as the [Teletype Model 33](/source/Teletype_Model_33), operated on a lower 20&nbsp;mA current level and most early [minicomputer](/source/minicomputer)s featured a 20&nbsp;mA current loop interface, with an RS-232 port generally available as a more expensive option.  The original [IBM PC](/source/IBM_PC) [serial port](/source/serial_port) card had provisions for a 20&nbsp;mA current loop.<ref name=appnote>http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf {{Webarchive|url=https://web.archive.org/web/20170420195811/http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf |date=2017-04-20 }}  Current Loop Application  Note, retrieved July 11, 2017</ref>

==Signaling conventions==
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).<ref name=PARR98/> This is done to ensure that on normal conditions there is always current flowing and in the event of a line being cut the flow stops indefinitely, immediately raising the alarm of the event usually as the heavy noise of the teleprinter not being synchronized, something that would not have been possible if the idle state  had been no current flowing.

==Electrical characteristics==
The maximum [resistance](/source/electrical_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](/source/RS-232) interface, and cannot be interconnected with voltage-type inputs without some form of level translator circuit.<ref name=appnote/>

For full-duplex communication between two devices, two pairs of wires would be used. There is no common standard for current loop interfaces, so details such as timing, connectors, wire color codes, and so on, are all application specific.

==See also==
* [MIDI](/source/MIDI), a digital current loop interface limited to 5 milliamps and 5 volts.

==References==
{{Reflist}}

Category:Communication circuits
Category:Electronics standards
Category:Serial buses

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Adapted from the Wikipedia article [Digital current loop interface](https://en.wikipedia.org/wiki/Digital_current_loop_interface) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Digital_current_loop_interface?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.