Siemens SIMATIC S7-400 Reference Manual page 251

Circuit for resistance measurement
The following conditions apply when measuring the resistance and temperature with the SM
431; AI 8 x 14 Bit:
Table 5-50 Channels for resistance and temperature measurement of the SM 431; AI 8 x 14 Bit
Measuring type parameter
Resistance
(4-conductor connection)
Resistance
(3-conductor connection)
Thermal resistance
(linear, 4-wire connection)
Thermal resistance
(linear, 3-wire connection)
Circuit for reference junction compensation for thermocouples
If you select "RTD on Channel 0" as a reference junction for reference junction compensation
for thermocouples, the following applies:
Table 5-51 Thermocouple with reference junction compensation via RTD on channel 0
Measuring type parameter
RTD on Channel 0
Unused channels
Unused channels can usually be left open. Set the measuring range modules to position "A".
In measuring environments with strong interference, you can improve the module's immunity
to interference by short-circuiting the channels.
For unused channels, set the "measuring method" parameter to "deactivated". This reduces
the module's cycle time.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
5.20 Analog input module SM 431; AI 8 x 14 Bit (6ES7431-1KF10-0AB0)
Permissible for
channel n
0, 2, 4 or 6 You must disable the "Measuring type" parameter for channels n+1
(1, 3, 5, 7).
0, 2, 4 or 6 Reason: The connections of channel n+1 are used to supply the
resistance that is connected to channel n.
0, 2, 4 or 6
0, 2, 4 or 6
Permissible for
channel n
2 to 7 You must connect and configure on channel 0 a resistance thermom‐
eter with linearization, a 3 or 4-conductor connection in climatic
range. This means that channels 0 and 1 are assigned.
Reason: If channel 0 is to be used as the reference junction, a resist‐
ance-type sensor must be connected there to record absolute tem‐
peratures in the climatic range.
Analog modules
Condition
Condition
251