Siemens SIMATIC S7 Functional Safety Manual page 93

Some of the 24 VDC power inputs in an S7-1200 system are interconnected, with a common
logic circuit connecting multiple M terminals. For example, the following circuits are
interconnected when designated as "not isolated" in the technical specifications: the 24 VDC
power supply of the CPU, the power input for the relay coil of an SM, or the power supply for
a non-isolated analog input. All non-isolated M terminals must connect to the same external
reference potential.
Connecting non-isolated M terminals to different reference potentials will cause unintended
current flows that may cause damage or unpredictable operation in the PLC and any
connected equipment.
Failure to comply with this guideline could cause damage or unpredictable operation which
could result in death or severe personal injury and property damage.
Always ensure that all non-isolated M terminals in an S7-1200 system are connected to the
same reference potential.
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
WARNING
Fail-Safe CPU and signal module (SM) installation
4.2 Fail-Safe system electrical design rules
93