II - DCE FEL ČVUT v Praze

SIMATIC S7 Ex Analog Modules
3.2 Connecting Transducers to Analog Inputs
In this chapter
Line for
analog signals
Isolated analog
input modules
Isolation between
channels
3-22
Depending on the measurement mode, various transducers can be connected
to analog input modules:
Voltage sensor
Current sensor as
– 2-wire transducer
– 4-wire transducer
Resistant sensor
Shielded conductors twisted in pairs are used for the analog signals. (refer to
Section 1.8; Shielding and Measures to Counteract Interference Voltage)
In the isolated analog input modules there is no metallic connection between
M- of the measuring circuit and the M- terminal of the CPU.
Isolated analog input modules are used when there is to be a difference in
potential UISO between the reference point M- of the measuring circuit and
the M- terminal of the CPU. Take particular care to ensure that the difference
in potential UISO does not exceed the permissible value. If there is a
possibility that the permissible value for UISO may be exceeded or if you
cannot exactly determine the difference in potential, you must connect the
reference point M- of the measuring circuit to the M- terminal of the CPU.
This also refers to unused inputs.
When there is isolation between them, the channels are supplied individually
by transformers and the signals are transmitted by means of optocouplers.
Metallic isolation allows for high differences in potential between the
channels. In addition, very good values are achieved with regard to
interference voltage rejection and crosstalk between the channels.
SM 331; AI 4 x 0/4...20 mA features isolation between the channels.
To facilitate channel isolation, the SM 331; AI 8 x TC/4 x RTD is equipped
with optical semiconductor multiplexers which ensure a high common-mode
range of UCM 60 V DC between the channels. This represents a virtually
equivalent solution in practical applications.
Larger differences in potential are permitted when using the modules for
signals from non-Ex areas (refer to technical specifications of the modules).
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04