I'm studying the HART protocol. I know that it's an hybrid protocol with a digital signal superimposed in the 4-20ma signal. But how can we consider it? in the family of Analog signal or Digital signal?

Thank you in advance dear friends.

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I say, either, or both.

HART is definitely digital capable, but in practice HART is primarily used for configuration, troubleshooting, calibration and diagnostics.

Its use in or for control is limited by

- I/O availability - only DCS and high level PLCs have HART enabled I/O at a premium cost

- the relatively slow throughput rate, generally 2 updates/second

 

Thank you Mr.David for your additional informations, it will be very useful.

 

I personally agree with David_2: the real time signal used is analog 4-20 mA so it is really two separate signals on one pair of wires: one analog signal and one digital signal in the same cable

For a pure digital instrument technology look at FOUNDATION fieldbus. See the fieldbus brochure here:
http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf

Cheers,
Jonas