Siemens SIMATIC ET 200C, Manual

Page: 96 / 333

4-28
ET 200C Distributed I/O Station
EWA 4NEB 812 6119-02b
Wiring of ET 200C; AI 4/8 12 Bit (6ES7 144-0KH00-0KB0), Continued
The resistance of the resistance thermometer (Pt 100) is measured with the
aid of a 4-wire circuit.
The resistance thermometer receives a constant current Ic via terminals CH0
+
/ CH2
+
and CH0
–
/ CH2
–
. The voltage generated at the resistance thermome-
ter is measured via CH1
+
/ CH3
+
and CH1
–
/ CH3
–
. In this configuration,
voltage fluctuations on the constant-current lines do not falsify the results of
measurement. The measuring inputs are high-ohmic, so the voltage drop on
the measuring lines is negligible.
Note
If a resistance thermometer (Pt100) is used for floating measurement, the
maximum permissible common-mode voltage is U
CM
v "
1V and must not
be overshot relative to M
ANA
at any differential input.
The illustration below shows how resistance thermometers (Pt 100) are con-
nected in a 4-wire circuit for floating measurement:
M
ANA
1 2
OFF
1
234
5
678
ON
ON
1
234
56
78
ON
ON
CH0
COMP
CH1
CH2
CH3
CH4
CH5
CH6
CH7
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
Pt 100
Pt 100
.
.
S3
S2
S4
.
.
Shield
.
Figure 4-17 Connection to ET 200C; AI 4/8 12 Bit of Resistance Thermometers
(Pt 100) in 4-Wire Circuit for Floating Measurement
Resistance
Thermometer,
4-Wire Circuit
Mechanical and Electrical Installation