Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 172)

172 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 9 Troubleshoot Your Module
If the previously listed tasks fail to resolve your issue with incorrect voltage or
current readings on your module, use the following table.
Table 19 - 1756-OF8I Module - Troubleshoot Incorrect Input Voltage/Current Readings
Possible Cause of Incorrect
Output Description Recommended Action
Open wire One of the following:
• The load does not respond to the applied voltage/current output.
• A wire is disconnected from the module.
• The RTB is disconnected from the module.
One of the following:
• Check that the load is functioning properly.
• Check wiring to verify that all wires are connected.
• Check the RTB to verify that it is fully seated on the module.
Short-circuited output Reading minimum (downscale) can mean that there is a short-
circuited output. One of the following:
• Output to return short.
• Short to supply power.
• Short to ground.
Check wiring and correct terminations.
Electrical noise Erratic readings, that is, data fluctuating more than is typical, are a
cause of noise. An oscilloscope shows the magnitude of noise. • Disconnect all but one output to see if channels are affecting each
other, that is, there is bleed-over.
• Eliminate or suppress the effect of noise.
Employ hardware or software filters provided by the 1756-OF8I
module.
DC signal on top of the output signal Offset readings can be caused by a DC signal riding on top of the
output signal. An oscilloscope shows the magnitude of the offset.
Disconnect all but one input to see if channels are affecting each
other, that is, there is bleed-over.
Module is in calibration mode Incorrect readings can be a result of the module being calibrated
when the reading occurs. Make sure the module is not in calibration mode.
This symptom is module dependent, but in general, specific bits have
to be turned on to enable calibration.
Incorrect reading soon after module
installation One of the following:
• Incorrect readings at initial module installation and configuration
are often the result of installation and configuration errors.
• Incorrect readings at installation of an existing, previously-
working module are more likely the result of a hardware failure.
One of the following:
• If error is occurring on a new module installation and
configuration, make sure that those tasks were completed
corrected and correct any errors you find.
• If the error is occurring on an existing, previously-working
module, diagnose the hardware failure and correct it. If more than
one channel is experiencing the incorrect readings in this case,
disconnect all outputs except one. This can help determine if it is
external hardware or the module itself is the cause.
Load Compatibility The module is capable of driving current input load impedance of up
to 1000 ohms.
The module is capable of driving voltage input load impedance of
1000 ohms. Verify the loop impedance of the load driven by the module.
Invalid Clamp Values Check the Clamp values on the module configuration Limits tab.
They are not automatically changed when the scaling engineering
units are changed. If they are not changed a small value of voltage/
current out results. Change the Clamp values with respect to the scaling engineering
units.
Hold for Initialization If this feature is used, the output value does not change (hold value)
until the commanded value is at the output screw terminal within
0.1% of full scale. Verify that the Hold for Initialization feature is desired.
Channel Offset The software configured Channel Offset value is summed with
the output. Verify that the Channel Offset feature is desired.
Programming and Configuration One of the following:
• The output value is written to the incorrect data tag.
• The channel is not configured for correct range.
• The Signal and Engineering units are not set correctly.
If the incorrect output data is written from a secondary location, for
example, an HMI device, verify that the output base tag value is
correct in the controller.
Correct programming or configuration as needed.