Siemens Simatic M7-400 Reference Manual page 260

Analog Modules
Abbreviations and Mnemonics Used in the Figures Below
The abbreviations and mnemonics used in the figures below have the following
meanings:
M +:
M –:
M :
ANA
U :
CM
U :
ISO
Connection of Isolated Measuring Sensors
The isolated sensors are not connected with the local ground potential (local
ground). They can be operated free of potential.
With isolated sensors, potential differences might arise between the different
sensors. These potential differences can arise as a result of interference or the
local distribution of the sensors.
To ensure that the permissible value for U
EMC-affected environments, connect M– to M
connection.
Figure 5-7
Note
Do not connect M– to M
measurement and when connecting resistance-type sensors. This also applies to
inputs which are not used.
5-44
Measuring line (positive)
Measuring line (negative)
Reference potential of the analog measuring circuit
Potential difference between inputs and reference potential of
the M measuring circuit
ANA
Potential difference between M
Isolated
sensors
Connection required
for modules with
M
ANA
Connecting Isolated Sensors to an Isolated AI
when connecting two-wire transmitters for current
ANA
S7-400, M7-400 Programmable Controllers Module Specifications
and chassis ground
ANA
is not exceeded during use in heavily
CM
in modules with an M
ANA
M+
M–
M+
M–
M
ANA
U
ISO
Chassis ground
ANA
A5E00069467-07