Answer Posted / 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
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