A Troubleshooting
A.1 Troubleshoot HART communication
Symptom Possible cause Solution
"No Voltage Detected" message
is displayed.
The lead set is improperly
connected to the device.
Attach the lead set to the
device.
The Trex unit is connected to a
4-wire device or a wireless
device that is powered by its
battery.
None. This is expected
behavior.
There is intermittent
communication.
Insufficient loop current and
voltage at the device terminals.
Verify there is at least 4 mA and
12 VDC at the device terminals.
Noise on the loop, or noise or
signal distortion from the
control system. For example,
noise from the power supply
powering the devices, or front
end analog circuitry inside the
control system may be
distorting the HART signal.
An oscilloscope can help
determine noise or signal
distortion of HART signals.
Poor wiring. Check the wiring terminations
and exposed signal wire for
damage.
There is no communication
with device.
Insufficient loop resistance at
the HART frequencies.
Add an additional 250 ohm
resistor in series in the loop.
Place the leads across the
resistor and verify
communication occurs.
Insufficient loop current and
voltage at the device terminals.
Verify there is at least 4 mA and
12 VDC at the device terminals.
The AC adapter is attached to
the Trex unit.
Remove the AC adapter from
the Trex unit and try to connect
to the device.
Device may be set to a HART
address other than zero
(multidrop mode).
Change the polling mode to
Poll by Address.
Control system is
communicating HART, but the
Trex unit is not communicating
properly.
An internal fuse may be blown. Connect a meter capable of
measuring M ohms to the
HART or mA terminals. An
OPEN reading may indicate a
blown fuse.
User Guide Troubleshooting
September 2021
User Guide 179
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