In a HART loop, how is the digital communication delivered relative to the analog current?

In a HART loop, how is the digital communication delivered relative to the analog current?

• A. Shared on the same two wires on top of the analog signal
• B. Uses a separate twisted pair
• C. Replaces the analog current
• D. Not used

Answer: (A)

In a HART loop, the digital communication rides on the same two wires that carry the analog 4–20 mA signal. The loop current continues to encode the process variable, while a small, high-frequency FSK signal is superimposed on that current to carry digital data. This lets the host and field device exchange information without needing a separate pair of wires, and without disturbing the analog measurement—the digital portion uses two tones at higher frequencies (around 1 kHz and 2 kHz) that sit on top of the DC loop current within the loop’s power and signal limits.

In a HART loop, the digital communication rides on the same two wires that carry the analog 4–20 mA signal. The loop current continues to encode the process variable, while a small, high-frequency FSK signal is superimposed on that current to carry digital data. This lets the host and field device exchange information without needing a separate pair of wires, and without disturbing the analog measurement—the digital portion uses two tones at higher frequencies (around 1 kHz and 2 kHz) that sit on top of the DC loop current within the loop’s power and signal limits.