Which statement best describes compatibility of HART with standard 4-20 mA loops?

• A. It transmits digital data on the same two wires that carry the 4-20 mA signal
• B. It requires a completely separate bus for digital data
• C. It only uses digital data and eliminates the 4-20 mA signal
• D. It uses a wireless link for digital data

Answer: (A)

HART is built to work on the same two wires used by a standard 4-20 mA loop, so the digital communication rides on top of the analog current rather than replacing or duplicating it. The transmitter’s current still carries the process variable as 4-20 mA, while a higher-frequency digital signal is superimposed on that same pair of wires. A HART-capable instrument or communicator separates the slow-varying analog current from the fast digital data, allowing both to coexist without rewiring. This means you can use the existing loop wiring for power and measurement and still exchange digital information about configuration, diagnostics, and status.

Because of that, the other ideas don’t fit: a completely separate bus would require new wiring, which defeats the purpose of compatibility with existing loops; eliminating the 4-20 mA signal would lose the actual process measurement; and while HART Wireless exists, the standard wired HART approach for compatibility uses the two-wire loop, not a wireless link by default.