Siemens Simatic S7-400 - Manual (Page 473)

M7-400 Expansions
12-3
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Behavior on the S7-400 Backplane Bus
The expansion modules can be accessed via the relevant programmable module
and not via the S7-400 backplane bus.
e.g. FM 456-4 e.g. EXM 478
Expansion
socket
Expansion
connector
Figure 12-1 Positions of Expansion Socket and Expansion Connector
Maximum Expansion
Figure 12-2 shows the maximum possible expansion using expansion modules for
a CPU 486-3, CPU 488-3, or FM 456.