Siemens Simatic S7-400 - Manual (Page 266)

Analog Modules
5-50
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
5.11 Connecting Resistance Thermometers and Resistors
Note
The necessary connecting cables, which result from the potential connection of the
analog input module and the sensors, are not drawn in the figures shown below.
In other words, you must continue to take note of and implement Section 5.8 with
its generally applicable information for connecting sensors.
Abbreviations and Mnemonics Used in the Figures Below
The abbreviations and mnemonics used in the figures below have the following
meanings:
I
C+
: Constant-current lead (positive)
I
C–
: Constant-current lead (negative)
M
+
: Measuring line (positive)
M
–
: Measuring line (negative)
Connecting Resistance Thermometers and Resistors
The resistance thermometers/resistors are wired in a four-conductor,
three-conductor or two-conductor connection.
With four-conductor and three-conductor connections, the module supplies a
constant current via terminals I
C +
and I
C –
so that the potential drop occurring on
the measuring cables is compensated. It is important that the connected constant
current cables are directly connected to the resistance thermometer/resistor.
Measurements with four-conductor or three-conductor connections supply a more
precise measuring result due to compensation than measurements with a
two-conductor connection.