Siemens SIMATIC ET 200S Manual page 402

Analog Electronic Modules
13.2 Fundamentals of analog value processing
13.2.1 Connecting measuring sensors
Introduction
You can connect different measuring sensors to the analog input modules,
depending on the type of module involved:
• Voltage sensor
• Current sensors as:
– Two-wire measuring transducer
– Four-wire measuring transducer
• Resistance-type sensor
In this chapter you will find out how to connect the measuring sensors and what to
watch for when doing so.
Lines for analog signals
You should use shielded and twisted-pair lines for the analog signals. This reduces
the effect of interference. You should ground the shield of the analog lines at both
ends of the line. If there are differences in potential between the ends of the line, a
compensating current flows via the shield that can interfere with the analog signals.
If this is the case, you should only ground the shield at one end of the line.
Analog input modules
In the case of the analog input modules there is electrical isolation:
• Between the logic circuitry and backplane bus
• Between the load voltage and the channels. The following differences exist:
– No isolation: Connection between M
– Isolation: No connection between M
(U
13-18
)
ISO
and the central grounding point
ANA
and the central grounding point
ANA
ET 200S Distributed I/O System
EWA-4NEB780602402-09