Why we use 250ohms resister
Why we use 250ohms resister ?
HART communications requires a minimum of 230 ohms to develop a sufficient voltage drop for the 1200 baud FSK HART signal to be ‘seen’ by a HART communicator/modem. The capacitance in a DC power supply acts as a low pass filter which filters/strips off the FSK HART data. Additional loop resistance is needed for HART communications to take place. If there is insufficient loop resistance, the HART communicator/modem will fail to establish HART communications. In normal field work, it is assumed that the analog input to which the field instrument is wired will provide the minimum resistance necessary.
Most DCS analog inputs have an input resistance of 250 ohms. But not all devices do. And when bench testing/configuring HART instruments it is not uncommon to connect a DC power supply, but not an analog input. A HART communicator/modem will not work with just a power supply powering a HART field instrument. The loop requires a minimum of 230 ohms. A 250 ohm resistor is commonly used for a bench test/configuration because they’re commonly available in the instrument shop.
Credits - Carl Ellis