Siemens S7-400 Benutzerhandbuch

5-44

S7-400, M7-400 Programmable Controllers Module Specifications

A5E00069467-07

The abbreviations and mnemonics used in the figures below have the following
meanings:

M +:

Measuring line (positive)

M –:

Measuring line (negative)

M

ANA

:

Reference potential of the analog measuring circuit

U

CM

:

Potential difference between inputs and reference potential of 
the M

ANA 

measuring circuit

U

ISO

:

Potential difference between M

ANA

 and chassis ground

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

CM

 is not exceeded during use in heavily

EMC-affected environments, connect M– to M

ANA

 in modules with an M

ANA

connection.

M+

M–

U

ISO

M+

M–

M

ANA

Connection required
for modules with
M

ANA

Chassis ground

Isolated 
sensors

Figure 5-7

Connecting Isolated Sensors to an Isolated AI

Do not connect M– to M

ANA

 when connecting two-wire transmitters for current

measurement and when connecting resistance-type sensors. This also applies to
inputs which are not used.