Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 381)

Publication 1756-UM009C-EN-P - December 2010 381
Additional Specification Information Appendix E
How to Deal with Incorrect Thermocouple Temperature Readings
The first thought when an incorrect temperature reading is reported back in a
thermocouple input module is that the module is out of calibration. This is
typically not the case, particularly if the module has just been installed
out-of-the-box.
All thermocouple input modules are shipped factory calibrated so it is unlikely
that they would need to be calibrated on installation.
To determine the cause of the incorrect reading, the nature of the incorrect
reading must be discerned first. The module:
1. Always reads maximum.
2. Always reads minimum.
3. Reads erratic (data jumping around).
4. Reads with an offset over the entire range.
In general, if incorrect readings appear on a new install then checking for
proper installation and configuration would typically prevail as a cause versus
an existing working module where a hardware failure of some type (channel or
module) would be more likely the cause.
Also, if more than one channel is experiencing these symptoms, disconnect all
thermocouples except one. This can help determine if it is external hardware
or the module itself is the cause.
Before attempting to troubleshoot these symptoms, a great deal of work can
be saved by first, visually inspecting the module, and second, applying a
thermocouple emulator directly to the module input in question. Make sure
the module is powered and communicating based on the status indicators. Red
or flashing green status indicators signify a problem.
Make sure wiring is intact and correct and the cold junction sensors (CJS) are
installed correctly for the correct wiring arm, terminal base, or removable
terminal block. If all looks correct, then remove the thermocouple from the
channel in question and apply the emulator.
The emulator is designed to provide a voltage at the terminals equivalent to the
voltage expected for the thermocouple type it is emulating. If the temperature
reports back correctly then the module is performing as expected and the
thermocouple and wiring are suspect. If the emulator temperature is not
reporting back correctly, then the module hardware, configuration, or the
software application are suspect.