Why we use 250ohm resistor in HART protocol and why don’t we use it in fieldbus?

what is the purpose of 250ohm resistance in hart

HART works on (0-5v) DC WKT V=IR (20mA*250= 5 V) and another reason is HART baud rate(19600) is high compare to transmitters(9800), so in order to communicate baud rate mismatch will happen that’s why we reduce the baud rate of HART by adding resistor(250ohms)