HART (“Highway Addressable Remote Transducer”) is a communication protocol designed for industrial process measurement and control applications. It’s called a hybrid protocol because it combines analog and digital communication. It can communicate a single variable using a 4-20 ma analog signal, while also communicating added information on a digital signal.
The digital information is carried by a low-level modulation superimposed on the standard 4-to-20 mA current loop. The digital signal does not affect the analog reading because it’s removed from the analog signal by standard filtering techniques. The ability to carry this added digital information is the basis for HART’s key benefits.