Siemens SIMATIC S7-300 Series Reference Manual page 145

System ID (only CPU 318-2)
In the object properties of the "General" tab, you can assign a system ID when
configuring your CPU. This ID can be evaluated in the CPU user program (also
refer to the STEP 7 Online Help relating to the "General" tab).
MPI Addressing
CPU 318-2
The CPU addresses the MPI nodes within its
configuration (FM/CP) via the module start
address.
If FM/CP are in the central configuration of an
S7–300 with their own MPI address, the CPU forms
its own communication bus (via the backplane bus)
with the FM/CP, separate from the other subnets.
The MPI address of the FM/CP is no longer
relevant for the nodes of other subnets.
Communication to the FM/CP takes place via the
CPU MPI address.
Your S7-300 structure includes an FM/CP addressed via MPI. You want to replace
the CPU 312 IFM ... 316 with a CPU 318-2. An example is shown in Figure 4-1,
Page 4-4.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
CPU Function, depending on CPU and STEP 7 Version
CPUs 312IFM to 316-2DP
The CPUs address the MPI nodes within their
configuration via the MPI address.
If FM/CP are in the central configuration of an
S7–300 with their own MPI address, the FM/CP
and CPU MPI nodes are in the same CPU subnet.
4-3