A Made IT project

Current Loop

Introduction
Current Loop is a serial interface, which is used to transport data over long distances. There are three different versions:

Digital:

20 mA polar Current Loop
30 mA polar Current Loop
60 mA polar Current Loop
Analog:

4 to 20 mA (used for analog sensors)
Bipolar Current Loop

The 60 mA version was used by the military in the early 1960's. In the 60's and 70's the 20 mA became widely used and still is the most used Current Loop variant.
The benefits of Current Loop over other kinds of transmission before the invention of e.g. RS422/485 was the long distance and noise immune transmission of data. The 20 mA version is capable of sending 19k2 baud over 600 meters (2000 feet), which is impossible with RS232
For Current Loop there is no mechanical or electrical standard

Technical Details
Since a current is used to transmit data there is a need for a loop, hence the name. Each circuit (transmit and receive) has it's own loop.

As been shown in the above picture Current Loop consists of active and passive devices. In one circuit (2 or more devices) there may and must be only one active device all others need to be passive. This way it's possible to built a network by daisy-chaining several passive devices and one active device.

A Space is represented by the absence of current and a Mark is represented by a current running through the circuit.
Next to the full duplex version, there is also an half-duplex possibility. For pinning details see the next picture.

Resources:

Current Loop Surge Suppressors ">http://www.axiomatic.com/cntlop_w.html">Current Loop Surge Suppressors