in reply to: Re: Multidrop HART and Intrinsically Safe posted by Dariusz Piszer on 31 May 2004 at 14:54:35.
15 devices multidrop for Group IIB?
I think this is the problem, Dariusz: if you put the minimum necessary 230 ohm load in the loop with 60mA, it will drop 14V, leaving only 4V for the field devices (not enough!). So you would have to use something clever as a HART load. I think maybe you could use a 250 ohm resistor with a large inductor in parallel, so the HART impedance is high but DC resistance is low. (You would also need a small inviolable resistor in series with the inductor, to make it safe in an IS circuit.) As far as I know, no-one has ever tried this. If you get to try it, please report the result!
Of course, the normal 4 devices is good for Group IIC, which is what many users need, and there are IS power supply / isolator / HART repeaters available for IIC, which don't need the extra 250 ohms (it is in the isolator as the current-limiting component).