Siemens SIMATIC ET 200iSP Operating Instructions Manual page 387

Extension to a reference junction
From their point of connection, thermocouples can be extended using equalizing cables as far
as the reference junction (transition to copper wiring). The reference junction can also be a
ET 200iSP terminal module.
These compensating wires are made of the same material as the thermocouple wires. The
supply lines are made of copper. Ensure correct polarity when connecting.
Compensation by resistance thermometer at the 4 AI RTD
If thermocouples connected to the inputs of the 4 AI TC share a common reference junction,
compensate using a 4 AI RTD:
For the channels of the 4 AI TC module, you can select "1", "2" or "Internal" as the reference
junction number. If you select "1" or "2", the same reference junction (RTD channel) is always
used for all four channels.
In the following figure, the electronic module 4 AI RTD is parameterized for the measuring range
Pt100 climatic range. The isolated thermocouples are compensated externally by a resistance
thermometer connected to the 4 AI RTD (channel 0).
M+
M-
I +
C
I -
C
Figure 14-6
ET 200iSP
Operating Instructions, 11/2022, A5E00247483-AK
Measuring cable (positive)
Measuring cable (negative)
Constantcurrent cable (positive)
Constant-current cable (negative)
Compensation using 4 AI RTD
Analog electronic modules
14.9 Fundamentals of analog value processing
387