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we put it as a comensation for the cabling resistance normally available when the instrument is connected in the field due to the existance of cables and wiring , this is to prevent the analog signals from being ignored .
for the value of250 ohms this is to go with a standard1-5 volts and4-20 mA DC as per Ohms law V=IR , R= V / I , R=1/0.004 =250 ohms =5/0.02
The HART communicator is communicating with the transmitter through FSK protocol. Understanding this protocol, it is a communication in analog platform. Digitalization of the signal happens within the HART devices and not along the transmission line. The resistor's position in the loop is between the transmitter and the DC power supply. As we connect the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? Because the power supply has a LOW PASS CAPACITOR filter that basically squelches analog signals like ripples. Thus, if there is no resistance in between the PS and the TX that analog signal coming from the HART devices will only be filtered out. Explaining the value250: The resistance value is not necessarily be250 ohms. Maybe because of the idea that the controlling current of20mA will give us5 volt calibration if there is a loop resistance of250 ohms. Maybe not a good theory. Actually you can use a number of possible resistance values that will allow communication between HART devices. This value depends on the transmitter minimum operating voltage requirement. Simple OHM's LAW. For example, if the voltage requirement for a certain transmitter is12-24 volts use a resistor that will not give you a drop of below12 volts or else you will not deliver power to your transmitter. Most transmitters are supplied with24 volts and as per computation250 ohms to500 ohms gives a drop of almost18 v which is somewhat in between the operating voltage.
