The purpose of this current loop application note is to introduce the reader to the physical aspects of 20 mA current loop

Until the early 1960s, 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 60s, 70s, and early 80s, 20mA 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.