Siemens SIMATIC S7-300 Manual page 191

Connecting thermocouples via compensating box
If all thermocouples connected to the module's inputs share a common reference junction,
compensate the circuit as follows:
Figure 4-15
Note
To compensate the analog input modules, always use compensation boxes with a reference
junction temperature of 0 °C.
S7-300 Automation System Module data
Manual, 08/2006, A5E00105505-04
Connecting thermocouples to electrically isolated analog inputs via compensation box
Priniples of analog value processing
4.6 Wiring thermocouples
4-17