250 ohms Resistor for HART Communicator

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.


In addition to that, a 250 ohm resistor required in PLC or DCS Analog input cards to convert the loop current into equivalent voltage so that ADC converts it into equivalent binary code.

If we connect the Transmitter in the calibration lab then you need to connect a external 250 ohm resistor.

If we connect the HART communicator in field then you no need to connect this resistor because it is already available at PLC or DCS side (control room).