Which configuration is considered non-intrusive for Hart communication on a live loop?

Which configuration is considered non-intrusive for Hart communication on a live loop?

• A. In series with the loop
• B. Across the loop in the middle of the transmitter
• C. Across the power supply
• D. In parallel to the transmitter

Answer: (D)

In HART communication, non-intrusive means tapping the 4-20 mA loop without changing the loop current or voltage that actually carries the process signal. The best way to do this is to connect the HART interface in parallel with the transmitter, across the transmitter’s output terminals (the two loop wires). This high-impedance tap allows the digital signal to ride on top of the same loop current without loading the loop or disturbing the transmitter’s regulation. If you put the interface in series with the loop, across the power supply, or in other configurations that alter the loop’s impedance or supply conditions, you would change the current or voltage seen by the loop, which can skew the process measurement and affect operation. The parallel connection keeps the loop behavior intact while permitting the digital data to be transmitted alongside the analog signal.

In HART communication, non-intrusive means tapping the 4-20 mA loop without changing the loop current or voltage that actually carries the process signal. The best way to do this is to connect the HART interface in parallel with the transmitter, across the transmitter’s output terminals (the two loop wires). This high-impedance tap allows the digital signal to ride on top of the same loop current without loading the loop or disturbing the transmitter’s regulation.

If you put the interface in series with the loop, across the power supply, or in other configurations that alter the loop’s impedance or supply conditions, you would change the current or voltage seen by the loop, which can skew the process measurement and affect operation. The parallel connection keeps the loop behavior intact while permitting the digital data to be transmitted alongside the analog signal.