Siemens SIMATIC ET 200AL, System Manual

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Connecting transducers
4.6 Wiring thermal resistors and resistors
Analog value processing
74
Function Manual, 06/2014, A5E03461439-AC
4.6 Wiring thermal resistors and resistors
The module provides a constant current at terminals I
C
+ and I
C
- for resistance
measurements. The constant current is fed to the resistance to be measured where it is
measured as voltage drop. It is important to wire the connected constant current cables
directly to the thermal resistor/resistor.
Measurements with 4-wire or 3-wire connection compensate for line resistance and therefore
achieve greater accuracy compared to measurements with 2-wire connection.
Measurements with two-wire connection always acquire line resistance in addition to the
actual resistance; you must therefore allow for a significant loss of precision in the measuring
results.
The figures below give some connection examples.
4-wire connection of a thermal resistor
The voltage at the thermal resistor is acquired using high-impedance measurement via the
M
0
+ and M
0
- terminals. Make sure the wiring is correctly poled (I
C0
+ and M
0
+, and I
C0
- and
M
0
- at the thermal resistor).
Always wire the I
C0
+ and M
0
+ lines, and the I
C0
- and M
0
- lines directly to the thermal resistor.
Figure 4-7 Example: 4-wire connection of thermal resistor to an analog input module