Answer / carl_ellis

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.

Carl Ellis
Measure First

Answer / 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.

Answer / mmuhammad habibullah

hart comunicator read 1 to 5 volt thats why we use 250ohm resistance
in control loop this resistnace already aad in loop

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