Siemens SIMATIC S7-300 CPU Data: CPU 315-T-2 DP User Manual

Communication with the S7-300
5.4 S7 communication structure
Time sequence for allocation of S7 connection resources
When you program your project in STEP 7 , the system generates parameter assignment
blocks which are read by the modules in the startup phase. This allows the module's
operating system to reserve or allocate the relevant S7 connection resources. That is, for
instance, OPs cannot access a reserved S7 connection resource for PG communication.
The module's (CPU) S7 connection resources which were not reserved can be used freely.
These S7 connection resources are allocated in the order they are requested.
Example
If there is only one free S7 connection left on the CPU, you can still connect a PG to the bus.
The PG can then communicate with the CPU. The S7 connection is only used, however,
when the PG is communicating with the CPU.
If you connect an OP to the bus while the PG is not communicating, the OP can establish a
connection to the CPU. Since an OP maintains its communication link at all times, in contrast
to the PG, you cannot subsequently establish another connection via the PG.
5.4.3 Distribution and availability of S7 connection resources
Distribution of the S7 connections
The distribution of the S7 connections of the CPUs can be found in the following table:
Table 5-5 Distribution of the S7 connections
Communication service Distribution
PG communication
OP communication
S7 basic communication
In order to avoid allocation of the S7 connections being
dependent only on the chronological sequence in which various
communication services are requested, S7 connections can be
reserved for these services.
At least one S7 connection is reserved by default for the PG
and OP communication respectively.
In the table below, and in the technical data of the CPUs, you
can find the configurable S7 connections and the default
settings for each CPU. You "redistribute“ the S7 connections by
setting the relevant CPU parameters in
STEP 7 .
S7 communication
Other communication connections
(e.g. via CP 343-1, with a data
lengths of > 240 bytes)
Here you allocate the S7 connections which are still available
and not reserved for a specific service (PG/OP communication,
S7 basic communication).
S7-300 CPU Data: CPU 315T-2 DP
5-16 Manual, 12/2005, A5E00427933-02

Siemens SIMATIC S7-300 CPU Data: CPU 315-T-2 DP Specifications

General

Brand	Siemens
Model	SIMATIC S7-300 CPU Data: CPU 315-T-2 DP
Category	Control Unit
Language	English