Siemens SIMATIC S7-300 Series Reference Manual page 32

CPUs
Interface Resources for CPU 318-2 - Example Calculation
1. Two network transitions by routing on the CPU
Resources used:
- 2 connection resources of the MPI/DP interface are used;
- 2 connection resources of the DP interface are used;
- all 4 connection resources available to both interfaces
are used;
2. 4 connections for S7 basic communication and PG/OP communication with the
CPU as connection terminal point via MPI/DP interface
Resources used:
- 4 connection resources of the MPI/DP interface are used;
- all 4 connection resources available to both interfaces
are used;
Resources still availabe:
- 26 connection resources of the MPI/DP interface;
- 14 connection resources of the DP interface;
- 24 of the connection resources available to both interfaces
Data Consistency for Communication
An essential aspect of the transmission of data between devices is its consistency.
The data that is transmitted together should all originate from the same processing
cycle and should thus belong together, i.e. be consistent.
If there is a programmed communication function such as X-SEND/ X-RCV which
accesses shared data, then access to that data area can be co-ordinated by
means of the parameter "BUSY" itself.
However, with S7 communication functions not requiring a block in the user
program of the 31x CPU (as server), e.g. PUT/GET or read/write operations via
OP communication, the dimension of data consistency must be taken into account
during programming. The following differences between CPUs 312IFM to 316-2 DP
and CPU 318-2 must be taken into account:
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PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01