For

serial communications In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits ar ...

, a current loop is a communication interface that uses current instead of

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

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 used by the

Teletype Model 33 The Teletype Model 33 is an electromechanical teleprinter designed for light-duty office use. It is less rugged and cost less than earlier Teletype models. The Teletype Corporation introduced the Model 33 as a commercial product in 1963, after ...

and was particularly common on

minicomputer A minicomputer, or colloquially mini, is a type of general-purpose computer mostly developed from the mid-1960s, built significantly smaller and sold at a much lower price than mainframe computers . By 21st century-standards however, a mini is ...

s and early

microcomputer A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (P ...

which used these as

computer terminal A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. Most early computers only had a front panel to input or display ...

s. As a result, most

s also supported this standard into the 1980s.

History

Long before the

RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such as a compu ...

standard, current loops were used to send digital data in serial form for

teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point (telecommunications), point-to-point and point- ...

s. More than two teleprinters could be connected on a single circuit allowing a simple form of networking.E. A. Parr, ''Industrial Control Handbook'', Industrial Press Inc., 1998, , page 648 Older teleprinters used a 60 mA current loop. Later machines, such as the

, operated on a lower 20 mA current level and most early

s featured a 20 mA current loop interface, with an RS-232 port generally available as a more expensive option. The original

IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...

serial port A serial port is a serial communication Interface (computing), interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in Pa ...

card had provisions for a 20 mA current loop.http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf Current Loop Application Note, retrieved July 11, 2017

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). 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 for a current loop is limited by the available voltage. Current loop interfaces usually use voltages much higher than those found on an

interface, and cannot be interconnected with voltage-type inputs without some form of level translator circuit. 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.

References

{{Reflist Communication circuits Electronics standards Serial buses

History

Signaling conventions

Electrical characteristics

See also

References