Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 167)

Rockwell Automation Publication 1756-UM540A-EN-P - May 2014 167
Troubleshoot Your Module Chapter 9
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 RTD 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-IRT8I
module, such as the Notch Filter.
DC signal on top of the
thermocouple signal Offset readings can be caused by a DC signal riding on top of the
thermocouple signal. An oscilloscope shows the magnitude of the
offset. Disconnect all but one RTD 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 thermocouples except one. This can help determine
if it is external hardware or the module itself is the cause
Grounded RTD Due to the module's excitation current traveling on an RTD lead that
is grounded there can be errant ground paths which would cause
inaccuracies to the RTD measurements. Eliminate the ground or use a signal conditioner/isolator.
Lead Compensation An RTD sensor requires at least 3 leads to compensate for lead
resistance error, caused by resistance mismatch. The amount of error
eliminated depends upon the difference between the resistance
values of the lead wires. The closer the resistance values are to each
other, the greater the amount of error that is eliminated To verify that the lead resistance values match as closely as possible:
• use heavy gauge wire (16–18 gauge)
• keep lead distances less than 1000 feet
• use quality cable that has a small tolerance impedance rating.
Sensor Offset The software configured Sensor Offset value is summed with the
input data value. Verify that the Sensor Offset feature is desired.
Programming and Configuration One of the following:
• Input value read from correct data tag.
• Channel configured for correct range.
• Signal and Engineering units are set correctly and equal.
If the incorrect input data value is in a secondary location, for
example, an HMI device, verify the base tag value in the controller.
Correct module configuration as needed.
Table 17 - 1756-IRT8I Module - Troubleshoot Incorrect RTD Readings
Possible Cause of Incorrect
Reading Description Recommended Action