Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 164)

164 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 temperature
readings on your module, use the following table.
Table 16 - 1756-IRT8I Module - Troubleshoot Incorrect Temperature Readings
Possible Cause of Incorrect
Reading Description Recommended Action
Open circuit A thermocouple reading maximum (upscale) usually means that
there is an open circuit condition.
The module indicates this condition through the following:
• The FLT status indicator for the channel becomes steady red.
• The I.Ch[x].Overrange tag is set to 1.
x represents the channel number.
• If the sensor type is Temperature, input data from the channel
changes to the highest scaled temperature value associated with
the sensor type.
• If the sensor type is mV, the input data for the channel changes
to the scaled value associated with the overrange signal value.
One of the following:
• Check the wiring, terminations, and for an open thermocouple.
• Make sure the length of the thermocouple cable is within module
specifications.
Wire length that is too long has a higher impedance, and can be
interpreted as an open circuit.
Short-circuited input Thermocouple reading 0 mV can mean that there is a short-
circuited input.
In some applications, the thermocouple reading 0 mV is correct. Check wiring and correct terminations.
Electrical noise Erratic readings, that is, data fluctuating more than is typical, are a
product of noise. An oscilloscope shows the magnitude of noise. • Disconnect all but one thermocouple 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 thermocouple 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 dependant, but in general, specific bits have
to be turned on to enable calibration.
Temperature reading difference
between maximum and minimum
temperatures All input channels on a 1756-IRT8I module can use the same
configuration and measure the same ambient temperature.
A temperature reading difference between upper and lower channels
up to -13.33…-12.22 °C (8…10 °F). can cause incorrect readings. To improve the temperature reading, we recommend that you select
remote cold junction compensation and wire to a 1492-AIFM8TC-3
IFM module.
CJC sensor is defective or installed
incorrectly Offset readings can be a result of a defective CJS or incorrect CJS
properly. Check the module input data for a CJC sensor defective diagnostic bit.
Thermocouples also report back ambient temperature and provide an
accurate ambient temperature if the CJC sensor is healthy, wired
properly, and the module is operating within specifications.
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 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.