Use of 250ohm resistance in HART Communication?

Why is it necessary to use a 250ohm resistance in HART communication?

  • 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.
  • In reply to Jonas Berge:

    From Yokogawa YS150/YS170 datasheet:
    "When an analog input is received from a 4-20mADC current transmitter, a precision 250Ω
    (±0.1%) shunt resistor is required. Up to two are provided in the shipping container with the
    YS150/YS170 controller. The resistance tolerance is critical, since the resistor is used to
    accurately convert the DC current to an analog voltage (1-5VDC). If a different current
    transmitter is used (e.g., 10-50mADC), an appropriate precision resistor must be used to develop
    the 1-5VDC input. Install the shunt resistor across the appropriate analog input terminals with the
    field signal wiring."
    note: 250 ohms * 4 mA = 1 V; 250 ohms * 20 mA = 5 V
  • In reply to chung.tranduc89:

    Yes. Precision 250 ohm is critical for the analog signal. However, as far as the digital HART signal is concerned the exact value is not critical
  • In reply to Jonas Berge:

    Thank you for this information.