Siemens SIMATIC S7-300 Manual page 308

Analog modules
6.9 Analog input module SM 331; AI 8 x RTD; (6ES7331-7PF01-0AB0)
Caution
Any faulty wiring of 3-wire connections may cause unforeseeable module states and
hazardous plant states.
Wiring: 2, 3 and 4-wire connection for resistor and thermoresistor measurement
Wiring possible at both ends at channel 0 to channel 7
Note
The module's product status 02 must be configured with an unused channel of an active
group of channels in order to avoid incorrect measurements.
Figure 6-22 Wiring and block diagrams
①
②
③
④
⑤
⑥
6-74
4-wire connection
3-wire connection
2-wire connection
Digital-to-Analog Converter
Backplane bus interface
Analog-to-Digital Converter (ADC)
S7-300 Automation System Module data
Manual, 08/2006, A5E00105505-04