Siemens SIMATIC S7-400 Hardware And Installation Manual page 164

Starting Up
Figure 6-4
Covering Unused Submodule Slots
On delivery, all the submodule slots are secured with a submodule cover. The
cover is attached to the frame of the card slot with screws.
Leave unused submodule slots secured.
6-22
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Inserting an Interface Submodule in a CPU
S7-400 and M7-400 Programmable Controllers, Installation Manual
Guides
C79000-G7076-C424-01