Siemens SIMATIC S7-300 Reference Manual page 263

Four-conductor connection
With a four-conductor connection, you must not connect the fourth conductor
(the conductor is not used at all, see Figure 4-21).
4th conductor
not connected
Figure 4-21
Programmable Logic Controllers S7-300 Module Data
A5E00105505-03
Four-Conductor Connection of Resistance Thermometers to the SM 331;
AI 8 x 13 bits
S–
ADC
M+
M–
Analog Modules
Backplane
bus
4-57