In Hart communication, how is information transmitted on the loop besides the main variable?

• A. A separate dedicated data line
• B. Wireless channel within the loop
• C. Digital signals superimposed on the loop current
• D. Only alarms and diagnostics

Answer: (C)

Digital signals are superimposed on the loop current. In a Hart loop, the 4-20 mA current carries the process variable, but a higher-frequency digital modulation rides on top of that same two-wire path, enabling two-way communication without a separate data line. This overlay lets the master query the device, read variables, and retrieve diagnostics while the analog current continues to represent the measured value. The digital channel is designed to coexist with the analog signal, so it doesn’t disturb the loop current. There’s no separate data line and no need for wireless communication, and the digital data goes beyond just alarms and diagnostics to include configuration and status information.