Siemens Automation System S7-400 - Manual (Page 104)

Networking
5-6
Automation System S7-400 Hardware and Installation
A5E00850741-01
PG Access
A CPU exchanges data with other systems using communication mechanisms, for
example with other programmable controllers, with operator interface stations (OP,
OS) or with programming devices (see Figure 5-2).
CPU
SIMATIC NET
MPI
PG OS PLC
PG OS
Slave
PG OS PLC
DP network
Figure 5-2 Data Exchange
Process communication, including communication services for data exchange
between programmable controllers (PLC - PLC) and between programmable
controllers and operator interface stations (PLC - OS/OP), has priority in CPUs
over communication between programming devices and CPUs.
The CPUs have different characteristics. One of these characteristics is their
communication performance. If the communication resources of a CPU are
occupied completely by process communication, this can severely hamper access
to the CPU from the programming device.