Answer / 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

Answer / nilesh bhatt

Hart communicator work wen it gets minimum 5 volt and when it connects with 250 ohm resistance it gets excited and works online or offline.

Answer / saurabh singh

1-5 volt standard voltage for hart operation.
4-20 ma standard current signal
When 4ma* 250v= 1 volt
20ma* 250v = 5 volt
At 5 volt hart comes in operation.

Answer / nitesh kumar

Because  250 ohm resistance converted 4-20 mA  standard current signal when we connect 250 ohm resistanc in parallel of 1- 5 volt supply..

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