Siemens Simatic S7-400 - Manual (Page 537)

Interface Submodules
13-41
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Connection of Resistance Thermometers (e.g. Pt 100) and Resistances
The resistance thermometers/resistances are measured in a 4-wire connection. A
constant current I
C
which can be set is supplied to the resistance thermometers/
resistances via one analog output QI each. The voltage arising at the resistance
thermometer/resistance is measured over the M
+
and M
-
terminals. This achieves a
high degree of accuracy in the measured results for 4-wire connection.
The lines carrying the constant current are to be run in parallel to the measuring lines
and are not to be connected together until the terminals on the resistance. Voltage
drops on the constant current lines will then not corrupt the measured result.
Three-wire connection is not possible in the case of the IF 961-AIO interface
submodule.
Figure 13-18 shows the connection of resistance thermometers/resistances with a
single current supply via one analog output each.
I
C
L
+
MV
0+
M
0-
QI
0+
+24 V
M
I
C
MV
1+
M
1-
QI
1+
0 V
Figure 13-18 Four-Wire Connection of Resistance Thermometers/Resistances with One
Constant Current Source Each
Figure 13-19 shows the connection of resistance thermometers/resistances with a
common current supply via only one analog output. Please take account here of
the maximum permissible load impedance for analog outputs and the maximum
permissible common mode voltage (V
CM
).
I
C
L
+
MV
0+
M
0-
QI
0+
+24 V
M
MV
3+
M
3-
0 V
Figure 13-19 Four-Wire Connection of Resistance Thermometers/Resistances with a
Common Constant Current Source