HART Protocol And 4–20 MA Loop Communication Fundamentals Practice Test

Active transmitters are self-powered and control the current in the loop to represent the measured value. The input device that reads that current should not supply power or try to regulate the loop itself—it's a passive device that simply senses the current or the resulting voltage drop. Connecting a self-powered transmitter to a passive input allows the transmitter to drive the loop current without interference, ensuring accurate 4–20 mA signaling. If you were to connect the active transmitter to an active loop power source or to an input that also provides power, the two sources could fight each other, upsetting the loop current and leading to incorrect readings or damage. Similarly, a passive transmitter without its own power would rely on the loop power, which isn’t the scenario described for an active transmitter.