Siemens SIMATIC Manual page 256

Principles of analog value processing
4.6 Wiring and connecting thermocouples
Connection of the reference junction
If all thermocouples connected to the module's inputs share a common reference junction,
compensate the circuit as follows:
Figure 4-16
Additional information
You can find additional information on the compensating box and connection to an analog
module on the Internet. (https://support.industry.siemens.com/cs/ww/en/view/18272332)
256
Connection of thermocouples to reference junction
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ