Siemens SIMATIC S7-300 Hardware And Installation Manual page 21

Changing the configuration of an existing SIMATIC S7 system
Proceed as described above if you want to change an existing system
configuration.
When retrofitting signal modules, also proceed in a configuring - 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 specific module in the manual: SIMATIC S7-300 PLCs,
Module Data Reference Manual.
S7-300 Programmable Controller Hardware and Installation
A5E00105492-01
Quick Guide
2-5