Loops - Emerson 375 User Manual

5-2 Troubleshooting

Loops

Verify the HART loop current and voltage on the
field device. Almost all field devices need at least
4mA and 12VDC to operate properly.
If there are multiple field devices on a multidrop
loop then set the 375 Field Communicator to digital
polling mode.
Inspection of the loop wiring can often times reveal
problems. The shield is normally grounded at one
end only, which is usually the control system.
Verify that 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 it's 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.