Ams trex™ Device Communicator

Sample values
Loop Current Flow Minimum voltage for 250 ohms.
mA
Volts
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.
A.2
Troubleshoot fieldbus communication
Symptom
Possible causes
Solution
No communication with the
device.
The Trex unit is connected to a
fieldbus segment with a host
system and the device on the
same segment is being
commissioned using the host
system.
The Trex unit is unable to
communicate with this device
until you go offline and then
back online.
An internal fuse may be blown. Connect a meter capable of
measuring M ohms to the FF
terminals. An OPEN reading
may indicate a blown fuse.
Unable to change the address
of a device.
The Trex unit is unable to
change the address of a device
that is currently the LAS.
Establish communication by
putting the Trex unit on the
fieldbus segment first and then
adding one or more devices.
By putting the Trex unit on first,
it remains the LAS and allows
the address change.
User Guide 
Troubleshooting
September 2021
User Guide 
181

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