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

Digital data in HART rides on the 4–20 mA loop using frequency shift keying, where two tones carry the bits. The two tones chosen for this standard are 1200 Hz and 2200 Hz. When digital data is transmitted, the loop is driven to one of these frequencies to represent a binary state, and the other frequency represents the opposite state, allowing reliable detection by the receiver while the analog current remains on the same pair of wires. The tones are widely spaced and well above mains frequencies, which helps minimize interference and makes the digital signal robust on the loop. Other frequency pairs are not defined for HART digital signaling, which is why they aren’t correct.