Siemens SIMATIC S7-300 Hardware And Installation Manual page 22

Installation Order
Basic rules
An S7 system can be used in many different ways, so we can only provide basic
rules for the electrical and mechanical installation in this section.
You must at least keep to these basic rules if you want your S7 system to operate
correctly.
Modifying the structure of an existing S7 system
If you are modifying the structure of an existing system, please apply the steps
mentioned above.
If you are adding an additional signal module, you should also follow this
configuration - mounting - wiring - etc. sequence. In this case, however, you only
have to consider information that is relevant to the new module.
Cross-reference
Also note the description of your specific module in the SIMATIC S7-300 PLCs
manual and the Module Data Reference Manual.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02