A serial transmission method that uses a closed loop. It originated with teletype machines that transmitted 20 mA (milliamperes) of current for a 1 bit and the absence of current for a 0 bit. A 60 mA loop is also used for digital signals. A common analog current loop is 4-20 mA (4 to 20 mA), which is widely used for process automation. Current loops provide a better signal-to-noise ratio (SNR) than voltage-based systems. Contrast with
RS-232. See
4-20 mA.