What best describes HART communication?

• A. A digital-only protocol on a separate wire pair
• B. A hybrid protocol sending digital data over the same two wires as a 4-20 mA analog signal
• C. A wireless protocol for process data
• D. A protocol that replaces 4-20 mA with digital signaling

Answer: (B)

HART communication is a hybrid approach that preserves the existing 4-20 mA analog loop and adds a digital data channel on top of it. The process variable continues to be carried by the analog current in the loop, while digital information such as device configuration, diagnostics, and status is transmitted simultaneously over the same two wires. This digital layer is typically implemented with frequency-shift keying, so the two signals share the line without interrupting the continuous analog signal.

Why this is the best description: it captures the essential idea that HART does not replace the analog loop or require separate wiring. It provides digital data alongside the analog measurement, enabling configuration and diagnostics without sacrificing the ongoing process signal. The other notions—being digital-only on a separate pair, being purely wireless, or replacing 4-20 mA with digital signaling—do not reflect how HART actually operates in practice.