How does HART overlay digital communication on a 4-20 mA loop?

How does HART overlay digital communication on a 4-20 mA loop?

• A. It superimposes a digital, bidirectional signal (FSK at about 1200 baud) on top of the DC current.
• B. It replaces the analog current with a digital waveform whenever data is exchanged.
• C. It uses a separate wired network in parallel to the loop for data.
• D. It uses wireless transmission to carry digital data alongside the loop.

Answer: (A)

HART delivers digital communication by riding a digital signal on the existing 4-20 mA loop. It overlays a bidirectional digital signal on top of the DC loop current, using frequency-shift keying to transmit data at about 1200 baud. The analog current continues to carry the process variable, so the loop remains powered and the measurement is uninterrupted. This approach means there’s no replacement of the analog signal, no separate parallel network, and no reliance on wireless transmission—the digital data travels alongside the loop current, accessible to both master and device.

HART delivers digital communication by riding a digital signal on the existing 4-20 mA loop. It overlays a bidirectional digital signal on top of the DC loop current, using frequency-shift keying to transmit data at about 1200 baud. The analog current continues to carry the process variable, so the loop remains powered and the measurement is uninterrupted. This approach means there’s no replacement of the analog signal, no separate parallel network, and no reliance on wireless transmission—the digital data travels alongside the loop current, accessible to both master and device.