Emerson 375 User Manual page 88

Troubleshooting
5-2
HART Loops
Verify the HART loop current and voltage on the field
device. Almost all field devices need at least 4 mA and
12V DC to operate properly.
If there are multiple field devices on a multidrop loop, set
the 375 Field Communicator to Poll by Address. See
"Changing the HART Polling Option" on page 3-17 for
more information on polling options.
Inspecting the loop wiring can often reveal problems.
The shield is normally grounded at one end only, which
is usually the control system.
Verify the loop has a minimum of 250 ohms resistance. If
there is not an external resistor, then, normally, the total
resistance is the resistance of the process-indicating
device monitoring the loop. Check the manuals for the
process-indicating device to determine its loop
resistance. If the resistance of the process-indicating
device is unknown, it can be determined by knowing the
current flow in the loop and the voltage across the
process-indicating device.
The resistance can be calculated using the formula,
Resistance = Voltage / Current. The 375 Field
Communicator can be used to measure the voltage; see
"DC Voltage Measurement (HART Terminals)" on
page 3-22.