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Troubleshooting
80
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 475 Field
Communicator to Poll by Address. See "Changing the HART polling
options" on page 53 for more information on polling options.
Inspection of 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 475 Field Communicator can be used to
measure the voltage; see "DC voltage measurement (HART
terminals)" on page 57.
Sample values
Loop Current Flow Minimum voltage for 250 ohms.
4 mA 1 Volt
8 mA 2 Volts
12 mA 3 Volts
16 mA 4 Volts
20 mA 5 Volts
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.
If an oscilloscope is available, use it to look for noise on the loop. Use
a scope with differential mode capability or a battery powered/isolated
scope to avoid grounding one side of the loop. Noise with a frequency
of 500 Hz to 10,000 Hz is of particular interest, as this represents
frequencies near the HART frequencies of 1200 and 2200 Hz.
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