industrial//instrumentation//HART

HART (Highway Addressable Remote Transducer) is an industrial communication protocol that superimposes a digital signal on a traditional 4-20 mA loop, and it is used to read configuration and diagnostics from field instruments over their existing wiring. The analog current still carries the primary variable; the digital layer carries configuration, diagnostics, secondary variables and device identification.

HART (Highway Addressable Remote Transducer) is an industrial communication protocol that superimposes a digital signal on a traditional 4-20 mA loop, and it is used to read configuration and diagnostics from field instruments over their existing wiring. The analog current still carries the primary variable; the digital layer carries configuration, diagnostics, secondary variables and device identification.

The digital layer is frequency-shift keying at 1,200 bit/s, a small tone of 1,200 Hz for a one and 2,200 Hz for a zero, about half a milliamp in amplitude, whose average is zero: the analog input, which filters well below those frequencies, does not see it. HART was developed by Rosemount in the 1980s and is now maintained by the FieldComm Group.

It let plants modernize instrumentation without replacing the wiring: the same two wires, the same PLC analog input for the process value, and a digital conversation for everything else. The analog current keeps its meaning and its speed, the digital overlay adds slow, rich data, and neither replaces the other. That is why it was central in the transition from purely analog to intelligent instruments in the 1990s.

A 1998 HART transmitter is exactly the kind of device a gateway still has to translate today, next to Modbus meters and PROFIBUS devices (edge gateway).