Siemens SIMATIC S7-300 Series Reference Manual page 85

1.4.8 CPU 318-2
Special Features
S 4 accumulators
S The configuration of MPI interfaces can be changed: MPI or PROFIBUS DP
(DP Master).
S Configurable data areas (Process image, local data)
Information on differences between CPU 318-2 and other CPUs is found in
Chapter 4.1.
DP master or DP slave
You can operate the CPU 318-2 DP as DP Master or DP Slave in a
PROFIBUS-DP network. However, note that only one of the interfaces can be a
DP Slave. For details on PROFIBUS-DP characteristics of CPU 318-2 DP refer to
Chapter 2.
Definable Data Areas and Occupied Working Memory
In your CPU 318-2 configuration, you can change the size of the I/O process
image and the local data areas.
Increasing default values for the process image and local data requires additional
memory that would otherwise be available for user programs.
Take following dimensions into account:
S Input process image:
12 Byte in memory
Output process image:
12 bytes in memory
Example:
256 bytes in PII occupy 3072 bytes,
2047 byte in PIO already occupy 24564 bytes in memory.
S Local data
1 byte in memory
256 byte is default, depending on the priority class. With 14 priority classes
there are therefore 3584 bytes occupied in the working memory. With a
maximum size of 8192 bytes you can still allocate 4608 bytes, which are then
no longer available for the user program in the working memory.
Communication
You can transform the first CPU interface from MPI to DP interface operation. You
can operate the CPU as DP Master or DP Slave on this DP interface. Routing
reduces the maximum possible number of connections for each one of the two
interfaces by one connection per active PG/OP communication used by the CPU
318-2 as network node.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
1 byte PII occupies
1 Byte PIO occupies
1 local data byte occupies
CPUs
1-69