Emerson Exchange 365

It doesn't have to be exactly 250 ohm. Anything from 230 - 1100 ohm will do

The reason it is required is that the HART signal is transmitted as a current and received as a voltage. The resistor works as a shunt that converts the transmitted current to a voltage the other device receives. The second reason it is required is because a DC power supply will short circuit the AC HART signal. The resistor prevents this from happening.

Note that this resistor is only required for transmitters. It is not required for positioners.