II - DCE FEL ČVUT v Praze

3.3.3 Connection of Resistance Thermometers (e.g. Pt 100) and
Resistance Sensors
Lines for analog
signals
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
The resistance thermometers/resistant sensors are measured by means of a
4-wire connection terminal. The resistance thermometers/resistance sensors
are fed a constant current via terminals IC + and IC - . The voltage produced at
the resistance thermometer/resistant sensors is measured via terminals M+
and M- . In this way, a higher degree of accuracy of the measured results at
the 4-wire connection terminal are achieved.
Shielded lines twisted in pairs are used for analog signals. So as to reduce
interference influence .
Use a twisted-pair wire for the constant current line Ic+ and the sensing line
M+ in the 4-wire connection of thermal resistors and a second twisted pair for
Ic+ / M+. You will achieve a further improvement if you also twist these two
twisted-pair wires with each other (star-quad).
The information provided in Section 3.2 on differences in potential UCM and
UISO between the individual circuits still retains its validity.
Fig. 3-10 shows the connection of resistance thermometers to the isolated
analog input module SM 331; AI 8 x TC/4 x RTD.
I C
I C
CH0
CH1
.
.
.
CH6
CH7
M+
M-
I C+
I C-
M+
M-
IC+ IC- ADU
SIMATIC S7 Ex Analog Modules
P5V
M internal
Logic
Backplane
bus
Fig. 3-10 Connection of resistance thermometers to the isolated analog input module
SM 331; AI 8 x TC/4 x RTD
For the 2-wire, 3-wire connection, you must connect corresponding jumpers
in the module between M+ and IC + or M- and IC - . However, accuracy losses
in the measurement results should be expected as voltage drops at the
relevant supply lines cannot be recorded.
3-33