In tone transmission, how is the data carried?

In tone transmission, how is the data carried?

• A. A digital packet containing time-stamped data.
• B. A radio frequency carrier with amplitude encoding the value.
• C. A pure audible tone whose duration represents a measured value.
• D. A continuous stream of binary bits conveying the measurement.

Answer: (C)

Data in tone transmission is carried using frequency shift keying: a pair of discrete audio frequencies is used to represent binary values, and the sequence of those frequencies carries the digital information. In this scheme, the actual measurement value is conveyed by the analog current in the 4–20 mA loop, while the digital data is encoded as transitions between two tones superimposed on that same loop. The value is not read from how long a single tone lasts; it’s read from which frequencies appear and in what sequence. The other concepts—digital packets with time stamps, a radio carrier with amplitude encoding, a pure audible tone whose duration encodes the value, or a continuous stream of binary bits as the measurement—aren’t how HART tone transmission works.

Data in tone transmission is carried using frequency shift keying: a pair of discrete audio frequencies is used to represent binary values, and the sequence of those frequencies carries the digital information. In this scheme, the actual measurement value is conveyed by the analog current in the 4–20 mA loop, while the digital data is encoded as transitions between two tones superimposed on that same loop. The value is not read from how long a single tone lasts; it’s read from which frequencies appear and in what sequence. The other concepts—digital packets with time stamps, a radio carrier with amplitude encoding, a pure audible tone whose duration encodes the value, or a continuous stream of binary bits as the measurement—aren’t how HART tone transmission works.