Highway Addressable Remote Transducer (HART) communication is a commonly used mode of transmission for digital signals that are superimposed on the analog signal of a 420mA current loop.
The HART protocol is based on the phase continuous frequency shift keying (FSK) technique. Bit 0 is modulated to a 2200Hz sinusoidal signal, and bit 1 is modulated to a 1200Hz sinusoidal signal with a baud rate of 1200bps. These two frequencies can easily be superimposed on the analog current-loop signal, which is in the range of DC to 10Hz, without affecting either signal. This unique nature of the HART protocol enables simultaneous analog and digital communication on the same wire.
