Question about Hart communication

[Mickaleto]

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Hello to all on this forum.
Please, can you explain me why most HART devices needs a resistor of 250 ohm connected in series in order to be possible to establish communication with a device?
Since it's 4-20 mA current loop, resistor will obtain voltage drop 1-5V, I wonder why it needs to be 250 ohm or something near that value...
Thanks in advance.

 

[JörgK]

Have a look to the Website from the HART Communication Foundation. There are some articles to introduce to the basics of the HART communication.