in reply to: Re: Re: HART Multi-drop network posted by Zaheer on 16 July 2013 at 10:48:56.
I wonder if, perhaps, the voltage developed across the 250-ohm load (your 9V measurement) might be too much for the transmitters to work correctly. (Obviously, this is rather higher than you would get for a single field device, even at 20mA). I imagine the transmitter contains an internal current source, which would be fed from the internal positive rail, and might need at least a certain voltage across the current source, to provide the output transistor with a working supply.
Is there a spec for this (the maximum voltage that the field device can work into, on the HART signal loop)?