Siemens Simatic S7-300 - Page 176

Priniples of analog value processing
4.2 Wiring transducers to analog inputs
S7-300 Automation System Module data
4-2 Manual, 08/2006, A5E00105505-04
Cables for analog signals
Always use shielded twisted-pair cables to wire analog signals. This reduces interference.
Connect both ends of the analog cable shield to ground.
Any potential difference between the cable ends may cause an equipotential current on the
shield, and thus disturbance on analog signals. If this happens, you have to ensure a low-
ohm potential compensation and if necessary only ground the shield to a cable end.
Electrically isolated analog input modules
Electrically isolated analog input modules are not electrically interconnected at the reference
point of the measuring circuit (M
ANA
and/or M) and the M terminal of the CPU/IM153.
Always use electrically isolated analog input modules if there is a risk of a potential
difference V
ISO
developing between the reference point of measuring circuit (MANA and/or
M-) and the M terminal of the CPU/IM153 .
To ensure that the permissible potential difference V
ISO
does not exceed the permissible
value, use an equipotential bonding conductor between terminal M
ANA
and the M terminal of
CPU/IM153.
Non-isolated analog input modules
When working with non-isolated analog input modules you have to produce a low-ohm
connection between the reference point of measuring circuit M
ANA
and the M terminal of the
CPU or interface module IM 153. To do this, connect terminal M
ANA
to the M terminal of the
CPU or interface module IM 153. A potential difference between M
ANA
and the M terminal of
the CPU or interface module IM 153 may distort the analog signal.
Limited potential difference CMV
The permissible potential difference U
CM
(CMV/Common Mode) must not be exceeded. A
potential difference U
CM
may arise between
● the measurement inputs (M+ / M-) and the reference point of measuring circuit M
ANA
● the measuring inputs of the channels themselves.
The following diagrams show the actions required when wiring transducers.