Emerson AMS Trex User Manual page 183

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