Why we use 250 ohm resistor in series with HART protocol
communication?
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Answer Posted / mahalingam
The 250-ohm resistor is not placed in the loop for the idea of converting the current (4-20mA)from transmission to voltage (1-5V)for reception, and also the value 250 is not a special value for a loop resistance needed for HART communication.
Explaining first the true purpose of having a resistor in the loop:
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 value 250:
The resistance value is not necessarily be 250 ohms. Maybe because of the idea that the controlling current of 20mA will give us 5 volt calibration if there is a loop resistance of 250 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 is 12-24 volts use a resistor that will not give you a drop of below 12 volts or else you will not deliver power to your transmitter. Most transmitters are supplied with 24 volts and as per computation 250 ohms to 500 ohms gives a drop of almost 18 v which is somewhat in between the operating voltage
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