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why use to 250 ohm for hart protocol and why hart communicator not communicate without 250 ohm
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Answer Posted / carl_ellis
My friend Dan has an article on why HART needs a minimum
resistance in the loop to work.
http://blog.lesman.com/2011/10/06/add250-ohm-resistors-to-make-hart-work/#more-68
In summary, the loop power supply acts as a very low pass filter that tends to filter out the HART signal.
According to the HART Foundation spec, the 1200-2400 Hz HART signal needs a minimum (230 ohm) resistance in the loop to create a HART signal with enough amplitude to meet the sensitivity requirements of a HART modem receiver.
Carl Ellis
Measure First
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