Siemens SIMATIC S7-400 Hardware And Installation Manual page 57

Default Addressing
Under certain conditions, the CPU can handle the assignment between logical
address and geographical address for you (default addressing). The logical
addresses are then permanently assigned to the slots (default address).
Distributed I/Os are not taken into account.
Conditions for Default Addressing
The CPU assigns default addresses under the following conditions:
If only signal modules are inserted
(no IM, CP, FM inserted; no expansion racks connected)
If signal modules are used with their default settings (measuring ranges,
interrupt processing, etc.)
If modules are inserted in STOP mode or during power off
(modules inserted in RUN mode will not be taken into account, nor during the
change from RUN
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
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STOP RUN)
Addressing the S7-400
3-3