Siemens SIMATIC S7-1500 Manual page 15

Mounting
11. If you have empty slots on the active backplane bus, then you must provide these slots with
a slot protection (see Operation (Page 15)).
12. A "BACKPLANE" sticker is enclosed for the active backplane bus. Use this sticker to identify
the interface module. So you can see even with a fully equipped active backplane bus that
the I/O modules can be pulled without any reaction.
Slots of I/O modules
Note
Slots
The empty slots need to be provided with a slot protection. The slot protection is used to
secure mechanically and to protect the contacts.
① Slot protection
Figure 3-4 Active backplane bus equipped with modules and slot protection
14
Active backplane bus ST 1+12 slot, active backplane bus ST 1+8 slot, active backplane bus ST 1+4 slot
Equipment Manual, 07/2020, A5E46240468-AB