475 hart communicator resistor

kjroller

Senior Member
Location
Dawson Mn
Occupation
Master electrician
On a Analog output instrument in this case a fisher valve can you connect the Emerson 475 hart communicator direct to the valve (DVC6200) with no 250-ohm resistor? I know it doesn't hurt to use one but was wondering. Also I know why you use the 250 ohm resistor but am under the impression you can use up to 1100 ohms if need be which would operate between 4.4 and 22 vdc instead of 1-5vdc I understand you would rather use 250 ohm however I'm wondering if that is why you could hook up direct to a Analog output travel valve cause you would have a higher resistance internally through the instrument then 250 ohms at any given time. Let me know if I am on the right track or if anyone has any input. Thank you. I understand most analog input cards have 250 ohms built in strictly wondering on analog output.
 
Also as far as this is all concerned you can hook up a hart monitor 2 ways on 1st on the front and back of the resistor which measures your true 1 to 5 volts and also on your instrument with will read roughly 19 volts if it has a 5 volt drop from being in line with a resistor is I get the first way the Hart would read the voltage differential on the 1 to 5 volts from your 4 to 20 ma, but the second way is it measuring the difference from 19 to 24 from the power supply.?
 
Highway Addressable Remote Transducer (HART) protocol is fully backward-compatible to 4-20mA as you probably know its 4-20mA protocol with a superimposed digital signal on top, kinda like how DSL worked over phone lines.
I am no expert but I think the total loop resistance must be within the range 250-1100 ohm range for the digital communication to work, so you dont want to go over 1100 total or under 250 if you want the digital.
Without the the correct impedance the digital is lost but the 4-20 mA loop still works for real-time control and feedback.
 
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