| The purpose of this
current loop application note is to introduce the reader
to the physical aspects of 20 mA current loop communications.
Until the early 1960's, military
teleprinters used 60 ma current loops to communicate
over long distances. In 1962, the Model 33 teletype
was introduced and 20 mA current loop interfaces became
widely used. Throughout the 60's, 70's, and early 80's,
20 mA current loop interfaces were applied in many types
of equipment. Current loop interfaces became popular
at this time because they offered the most cost effective
approach to long distance, noise immune data transmission.
The 20 mA current loop is suitable for distances to
2000 feet at data rates up to 19.2k baud with careful
attention to interface design. It can be used at longer
distances when data rates are as low as 300 baud.
When the EIA 422 Standard (December
1978) and the EIA 485 Standard (April 1983) brought
forth the application of balanced differential digital
data transmission, the popularity of 20 mA current loop
rapidly diminished.
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