Siemens SIMATIC ET 200M Manual page 71

Installation
S Use the rails for "module change during operation" (only these can take the
active bus modules).
S Close unused slots with the backplane bus cover. Close the last bus module
with the bus module cover. The bus module cover is included with the BM
PS/IM and IM/IM bus modules. The backplane bus cover has to be ordered
(see Appendix A).
Example:
Bus module 2 40
(...7HB)
S If you use the ET 200M in an intrinsically safe area, use an explosion-proof
partition – preferably between the modules in the intrinsically safe area and the
modules in the non-intrinsically safe area.
4-6
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Bus module
cover
Backplane bus
cover
Distributed I/O Device ET 200M
EWA-4NEB780600602-06