Why is HART described as a hybrid protocol?

• A. It uses separate cables for digital bit transmission
• B. It sends digital data over the same two wires carrying the 4-20 mA analog signal
• C. It employs a time-multiplexed digital stream
• D. It uses wireless data for digital information only

Answer: (B)

HART is described as a hybrid protocol because it combines two modes of communication on the same two wires: the analog 4-20 mA current that continuously represents the process variable, and a digital data stream that is superimposed on top of that current. The digital information is sent without breaking the analog loop by using a frequency-based modulation (FSK), so the master can address devices, read configurations, and perform diagnostics while the process variable is still conveyed optically by the current. This coexistence of analog and digital signals on the same pair of wires is what makes HART a hybrid approach. The other ideas—using separate cables, time-multiplexing, or wireless data only—don’t describe how HART actually transmits data, so they aren’t correct.