Siemens SIMATIC Manual page 430

Analog modules
6.11 Analog input module SM 331, AI 6 x TC isolated (6ES7331-7PE10-0AB0)
Compensation of the thermocouple via an external Pt100 element with external thermoresistor
module
Because of the flexible structure of data record 2, you can use a separate external Pt100
element for each channel. In addition, the channels can also be grouped together by means
of the user application in such a way that they all use the same external Pt100. This simply
requires the same temperature value to be specified in DR2 for all the channels which use
the same reference temperature.
Note
Reference-junction compensation also results in an error in the temperature measured by
the isolated AI 6 x TC module. For this reason, great care must be taken when detecting the
reference junction. To keep this error to an absolute minimum, the reference-junction
temperature must remain as constant as possible.
Example: Transfer of a temperature value as a cold-spot temperature for channels 0 to 5, from a
thermoresistor module to AI 6 x TC:
Input address of AI 6 x TC: 238 (module address)
Input address of the thermoresistor: 128 (channel address)
Occupied memory:
M 20.0: Request bit for SFC "WR_REC"
M 20.1: Busy bit for SFC "WR_REC"
MW 22: Return value for SFC "WR_REC"
MW 0 to MW 12: Memory for data transfer (see table below).
UN M 20.0
UN M 20.1
SPB END
U M 20.1
SPB WRT
//Create the memory for data transfer
L B#16#01
T MB
L B#16#02
T MB
L PEW
T MW
T MW
T MW
T MW
T MW
430
// Check the request: new cold spot temperature
// Check whether WR_REC is busy
// Skip, if no transfer is required
ist
//Check whether WR_REC is busy
// Transfer the temperature in hundredthsl
// of a degree (Pt100 climatic))
0
// Transer the temperature in Kelvin
1
128 // Read out the input address of the used channel
of
// the thermoresistor module
2 // for channel 0 of AI 6 x TC
4 // for channel 1 of AI 6 x TC
6 // for channel 2 of AI 6 x TC
8 // for channel 3 of AI 6 x TC
10 // for channel 4 of AI 6 x TC
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ