Siemens SIMATIC S7 System Manual page 50

Installation
3.2 Power budget
Some of the 24 V DC power input ports in the S7-200 SMART 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 data sheets: the 24 V
DC power supply of the CPU, the power input for the relay coil of an EM, or the power supply
for a non-isolated analog input. All non-isolated M terminals must connect to the same
external reference potential.
Avoiding unwanted current flow
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 these guidelines could cause damage or unpredictable operation
which could result in death or severe personal injury and/or property damage.
Always ensure that all non-isolated M terminals in an S7-200 SMART system are
connected to the same reference potential.
Refer to the technical specifications for your particular CPU to determine the 24 V DC sensor
supply power budget, the 5 V DC logic budget supplied by your CPU and the 5 V DC power
requirements of the expansion modules and signal boards.
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WARNING
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART