HART Protocol and 4–20 mA Loop Communication Fundamentals Practice Test

HART carries its digital data on the analog loop by using two frequencies, a method called frequency-shift keying. The system switches between 1200 Hz and 2200 Hz to encode bits while the loop current stays in the 4–20 mA range. In the referenced mapping, the 2200 Hz tone is used to represent a binary zero. So whenever that tone is detected, it is interpreted as a 0, while the other tone encodes the opposite bit value. The framing of the data (start/end of frame) is defined by the protocol, but the 2200 Hz tone itself is what carries the bit value, not a frame boundary marker.