Why Need 250 ohms resistor in HART protocol


Michael Coats

The resistance does have to be in the circuit in order to connect to the transmitter with the HART communicator. The 250 Ohm resistor does indeed convert the 4-20 mA signal to 1-5 volts, but many DCS and PLC cards do that with a 250 ohm resistor across the inputs anyway. The specific reason the HART device requires a resistance is to develop the HART signal riding on the 4-20 mA DC signal so that the HART communicator can communicate with the transmitter. Prove it to yourself. Take a HART transmitter and connect it to a regulated 24 VDC power supply. In the loop place a decade box and set it at 250 Ohms. Place your HART communicator in the loop across the + and - terminals of the transmitter or across the decade box...either will work. You will get a device found message and be able to read your HART transmitter. Now reduce resistance on the decade box. Somewhere around 80 to 60 ohms the HART communicator will no longer communicate with the HART device. This is not an opinion, it is a fact.

>Sorry to disagree again with you Roy, but the 250 ohm
>resistor in the HART circuit is there for the ANALOG signal,
>not the digital signal. With 20 mA DC in the loop, typically
>full scale, the voltage drop across the precision 250 ohm
>resistor is exactly 5 volts. The A/D converter in the Analog
>Input card expects a range of 1-5 volts to interpret the
>analog transmission of a HART instrument. The resistor has
>nothing to do with the LOW Frequency digital signal (1200
>of the HART digital transmission.
>Changing the resistor to something else would certainly not
>effect the digital HART signal, but it would kill the
>calibration of the analog HART signal.
in HART ,it is not about converting 4-20 ma to 1-5 vdc.
in HART we can put any resistance in the range of 250-500 ohm to overcome the effect of capacitor located at power supply.