Which statement correctly describes how HART digital data is transmitted on a 4-20 mA loop?

Which statement correctly describes how HART digital data is transmitted on a 4-20 mA loop?

• A. A separate digital bus runs in parallel.
• B. A modulated signal superimposed on the DC loop current.
• C. Optical signaling along fiber.
• D. A second 4-20 mA loop dedicated to data.

Answer: (B)

HART carries digital information by superimposing a high-frequency modulation onto the existing DC 4-20 mA loop. The loop current still reflects the analog process value, but a digital waveform rides on top of that current, using high-frequency tones to encode data. The receiver separates the two: the DC component gives the analog measurement, while the superimposed HF signal is decoded to recover the digital data. This setup preserves compatibility with legacy 4-20 mA systems and avoids needing a separate wire for digital communication. In practice, the digital portion uses a form of frequency-shift keying (two tones) in the kilohertz range to represent bits.

HART carries digital information by superimposing a high-frequency modulation onto the existing DC 4-20 mA loop. The loop current still reflects the analog process value, but a digital waveform rides on top of that current, using high-frequency tones to encode data. The receiver separates the two: the DC component gives the analog measurement, while the superimposed HF signal is decoded to recover the digital data. This setup preserves compatibility with legacy 4-20 mA systems and avoids needing a separate wire for digital communication. In practice, the digital portion uses a form of frequency-shift keying (two tones) in the kilohertz range to represent bits.