Siemens SIMATIC ET 200C, Manual

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4-27
ET 200C Distributed I/O Station
EWA 4NEB 812 6119-02b
The resistance of the resistance thermometer (Pt 100) is measured with the
aid of a 2-wire circuit. A maximum of four Pt 100 thermometers can be con-
nected (1 resistance thermometer per channel group).
The resistance thermometer receives a constant current supply via terminals
CH0
+
/ CH2
+
and CH0
–
/ CH2
–
. Voltage drops on the measuring lines can
falsify the results of measurement. This type of measurement is of practical
value if the precision requirements are low.
The settings of the switches in DIL switch block S4 determine whether mea-
surement is floating or non-floating (see Section 4.4.2).
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 (Pt100) are con-
nected to channel 0 and channel 2 in a 2-wire circuit for floating measure-
ment:
M
ANA
1 2
OFF
1
234
56
7
8
ON
ON
1
23
4
5
6
78
ON
ON
CH0
COMP
CH1
CH2
CH3
CH4
CH5
CH6
CH7
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
Pt 100
Pt 100
S3
S2
S4
DIL Switch Block S4:
DIL Switches 1 to 4 for
2-Wire Circuit ON
DIL Switch Block S4:
DIL Switches 1 and 3
for 4-Wire Circuit OFF
Schirm
.
Figure 4-16 Connection to ET 200C; AI 4/8 12 Bit of Resistance Thermometers
(Pt 100) in 2-Wire Circuit for Floating Measurement
Resistance
Thermometer,
2-Wire Circuit
Mechanical and Electrical Installation