What type of signal does HART superimpose on the 4-20 mA signal?

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Study for the HART Protocol and 4–20 mA Loop Communication Test. Utilize flashcards and multiple-choice questions, with each question offering hints and explanations. Get ready for your exam!

Multiple Choice

What type of signal does HART superimpose on the 4-20 mA signal?

Explanation:
HART sends digital information by riding a high-frequency alternating signal on top of the 4-20 mA loop. The digital data are encoded in that AC waveform using frequency-shift keying, so binary information is conveyed by two tones rather than by changing a DC level. This means the main current still represents the process variable (the 4-20 mA), while the digital channel appears as small, fast AC variations superimposed on it. It’s not pulsed DC, since the data aren’t carried by discrete on/off current steps, and it’s not a plain DC digital signal, because the information travels via an alternating (AC) waveform.

HART sends digital information by riding a high-frequency alternating signal on top of the 4-20 mA loop. The digital data are encoded in that AC waveform using frequency-shift keying, so binary information is conveyed by two tones rather than by changing a DC level. This means the main current still represents the process variable (the 4-20 mA), while the digital channel appears as small, fast AC variations superimposed on it. It’s not pulsed DC, since the data aren’t carried by discrete on/off current steps, and it’s not a plain DC digital signal, because the information travels via an alternating (AC) waveform.

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