A-B 1756-L73XT module Rockwell ControlLogix 8MB memory control

Dealing 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 upon 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 multiple channels are 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 that the module is powered and communicating based on the status indicators. 

Red or flashing green status indicators signify a problem.

Make sure that the 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.

1756 ControlLogix-XT Controllers

The ControlLogix-XT™ controllers function in the same way as the traditional ControlLogix controllers.

The ControlLogix-XT products include control and communication system components that are conformally coated

for extended protection in harsh, corrosive environments:

• When used with FLEX I/O-XT™ products, the ControlLogix-XT system can withstand temperature ranges of -20…+70 °C (-4…+158 °F).

• When used independently, the ControlLogix-XT system can withstand temperature ranges of -25…+70 °C (-13…+158 °F).

• Customers who previously used the LXT chassis should now migrate to the K version of the chassis.

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