250 ohm vs 255 ohm resistors

[TwoBlocked]

I have not noticed, but I installed an ABB i\p last week and now I'm curious to what it was...

Why is it, on some Hart smart instruments, you have to add a resistor in line for the communicator to "see" the device?

An educated guess here; The Hart device needs to see a voltage difference between the comm + and the comm - to work. A resistor supplies that. I would think going across any voltage difference, such as the + and - on a loop powered transmitter would do that, but sometimes it doesn't. Don't know why.

Btw, I love Hart. Just too bad it is not the universal application it once was. Some devices have a proprietary protocol and a run-of-the-mill Hart reader won't work.

 

[gar]

230601-0927 EDT

Twolocked:

You need to understand what a current loop means and its practical application.

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[TwoBlocked]

230601-0927 EDT

Twolocked:

You need to understand what a current loop means and its practical application.

.

Your assumption is faulty. And it's TwoBlocked, a nautical term.