DC voltage measurement Possible cause Action
Higher than expected There may be more than one
voltage source on the fieldbus
segment.
Ensure there is only one voltage
source.
The power supply's voltage
setting may be set too high.
Verify the power supply's
voltage setting is correct. If
necessary, decrease the power
supply voltage.
Low-Frequency noise Possible cause Action
Higher than expected When the AC adapter is
attached to the Trex unit, it can
appear as low-frequency noise.
Disconnect the AC adapter from
the Trex unit to isolate it from
the local power grid.
The device or shield is not
properly grounded.
Ensure the device is properly
grounded.
Ensure the shield is properly
isolated from the terminal
housing. The shield should be
grounded at only one location
on the entire fieldbus segment
and it should be a high-quality
ground.
Repeat these checks for all
devices on the fieldbus segment.
The power supply is not
supplying a good, stable
voltage.
Ensure the power supply is
properly installed. Isolate the
power supply and check for
noise.
In-band frequency noise Possible cause Action
Higher than expected The device or shield is not
properly grounded.
Ensure the device is properly
grounded.
Ensure the shield is properly
isolated from the terminal
housing. The shield should be
grounded at only one location on
the entire fieldbus segment and
it should be a high-quality
ground.
A cable may be damaged. Visually inspect the cables and
test the resistance between the
two wires and each wire against
the ground.
Fieldbus Diagnostics application
User Guide 175
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