A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway
Addressable Remote Transmitter. The purpose of the HART standard was to create a way for
instruments to digitally communicate with one another over the same two wires used to convey a
4-20 mA analog instrument signal. In other words, HART is a hybrid communication standard, with
one variable (channel) of information communicated by the analog value of a 4-20 mA DC signal, and
another channel for digital communication whereby many other variables could be communicated
using pulses of current to represent binary bit values of 0 and 1.
The HART standard was developed with existing installations in mind. The medium for digital
communication had to be robust enough to travel over twisted-pair cables of very long length and
unknown characteristic impedance. This meant that the data communication rate for the digital
data had to be very slow, even by 1980’s standards.
Digital data is encoded in HART using the Bell 202 modem standard: two audio-frequency
“tones” (1200 Hz and 2200 Hz) are used to represent the binary states of “1” and “0,” respectively,
transmitted at a rate of 1200 bits per second. This is known as frequency-shift keying, or FSK. The
physical representation of these two frequencies is an AC current of 1 mA peak-to-peak superimposed
on the 4-20 mA DC signal. Thus, when a HART-compatible device “talks” digitally on a two-wire
loop circuit, it produces tone bursts of AC current at 1.2 kHz and 2.2kHz. The receiving HART
device “listens” for these AC current frequencies and interprets them as binary bits.