Why we use 250 ohm resister in series
hart communicator?
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Answer Posted / carl_ellis
Answer #2 is not correct because the HART communicator is
not an analog input, it does not use a 1-5Vdc signal. A
HART communicator gets and display a transmitter's primary
variable (or other variables) as a digital value; NOT as an
analog value, not as a 1-5Vdc signal developed across an
analog input's resistor. I repeat, HART communications is
NOT analog in nature, it is digital.
The HART Foundation specifies a minimum resistance of 230
ohms, as described in answer #1, in order to develop the
1200 baud HART signal, that rides on top of the DC loop
current signal.
It's easy to prove that 250 ohms (for 1-5Vdc) is not a
requirement for HART communications as a bench test.
Take a 2 wire loop powered HART transmitter, connect a DC
power supply and use a 330 ohm resistor in series in the
loop. The communicator will work and the primary process
variable will be correct. Then try a 470 ohm resistor.
The 470 ohm resistor will work, as well, and the primary
variable will be correct.
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