Hart Loops; A.1.1 Hart Loops - Emerson AMS Trex User Manual

Symptom
The Trex unit needs to supply
more current than the typical 4
mA to power a positioner. This
can occur when the positioner
needs more current to power
the CPU at faster speed for
diagnostics.
A.1.1

HART loops

Verify the HART loop current and voltage on the device. Almost all devices need at least 4
mA and 12V DC to operate properly. If there are multiple devices on a multidrop loop, set
the Trex unit to Poll by Address and ensure the Auto-Connect option in the Field
Communicator Settings is disabled.
Inspect the loop wiring to identify problems. The shield is normally grounded at one end
only, which is usually the control system.
Sample values
Loop Current Flow Minimum voltage for 250 ohms.
mA
4 mA
8 mA
12 mA
16 mA
20 mA
If the voltage across the process-indicating device is greater than the value listed for a
given current flow, the device has at least 250 ohms of internal resistance. If the voltage is
less than the value listed in the table for the given current flow, more resistance needs to
be added to the loop.
User Guide
Possible cause
Use the device connection
wizard in the Field
Communicator application to
increase mA.
Volts
1 Volt
2 Volts
3 Volts
4 Volts
5 Volts
Troubleshooting
Solution
1. In the Field Communicator
application, connect to the
positioner.
2. Change its polling address
to a non-zero value.
3. Disconnect the positioner,
and restart the Field
Communicator application.
4. Tap HART.
5. Tap Yes when prompted if
you want to provide power
from the Trex unit.
6. Tap Positioner.
7. Tap Yes when prompted to
increase current.
8. Select the current value.
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