HART Protocol and 4–20 mA Loop Communication Fundamentals Practice Test

In HART, digital data is sent on top of the 4-20 mA loop using frequency shift keying, where two tones carry the bit values. The 1200 Hz tone is one of those tones and it encodes a binary 1. The other tone (a higher frequency) represents binary 0, so the receiver can distinguish between 1s and 0s by which tone it detects. This approach keeps the data robust against variations in loop current, since it relies on frequency rather than amplitude. The start of frame and end of frame are not signaled by a single fixed tone; frame boundaries come from the overall framing structure and bit sequence defined by the protocol.