Which statement about 2-wire loop-powered devices is true?

Which statement about 2-wire loop-powered devices is true?

• A. They are powered by the same loop used for the signal, with power drawn from the loop.
• B. They require a separate power supply in addition to the loop.
• C. They cannot communicate using HART.
• D. They operate only in burst mode.

Answer: (B)

Two-wire loop-powered devices draw all operating power from the same 4-20 mA loop used for the signal. The loop current powers the transmitter, while the measured process signal flows through the same loop path. HART communication is layered on top of that loop current, sending digital data without needing a separate power source. So the statement that a separate power supply is required in addition to the loop is not correct. These devices can talk using HART while the loop remains powered, and they are not restricted to burst-only communication—the digital information rides continuously on the loop alongside the analog signal.

Two-wire loop-powered devices draw all operating power from the same 4-20 mA loop used for the signal. The loop current powers the transmitter, while the measured process signal flows through the same loop path. HART communication is layered on top of that loop current, sending digital data without needing a separate power source. So the statement that a separate power supply is required in addition to the loop is not correct. These devices can talk using HART while the loop remains powered, and they are not restricted to burst-only communication—the digital information rides continuously on the loop alongside the analog signal.