Where can a Hart modem be connected in a loop?

• A. Anywhere electrically parallel to the transmitter in the loop.
• B. In series with the transmitter.
• C. Across the loop current path outside the transmitter.
• D. Across the power supply.

Answer: (C)

In a 4-20 mA loop, the HART modem must tap into the same current path used by the transmitter without disturbing it. The digital communication is superimposed on the loop current, so the modem needs to be connected in parallel to the loop rather than in series or to the power supply. By placing the connection across the loop current path outside the transmitter, the modem can observe and insert the high-frequency HART signals without altering the transmitter’s control of the loop current. If it were in series, it would disrupt the loop regulation; if it were across the power supply, it wouldn’t access the loop current where the digital data actually rides. Therefore, the correct approach is to connect across the loop current path, in parallel with the transmitter, at the transmitter side.