Siemens SIMATIC S7-400 Hardware And Installation Manual page 115

Communication from PG/OP to Module without MPI
If one of the programming devices or operator panels connected to a multipoint
interface (MPI) communicates with an S7-400 module which does not have an MPI
connection (for example, SINEC CPs, FM 456 etc.), this module can be reached
via the CPU to whose MPI the programming device or operator panel is connected.
In this case, the CPU simply acts as an intermediary for the transfer. This type of
connection between a programming device or operator panel and a module only
communicating via the communication bus occupies two connection resources in
the CPU.
PG or OP
One connection
resource occupied
Figure 5-1
Maximum Number of Connections via MPI
When configuring the connections of a CPU 416 via MPI, remember to include the
PG connection in the maximum number of possible connections.
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
www.gkbpx.com
CPU
C bus
MPI
Two connection resources
occupied in the CPU
Communication between Programming Device/Operator Panel and a Module
without MPI
CP or FM
S7-400
station
One connection
resource occupied
Networking
5-5