Siemens Simatic S7-300 Technical Data Manual page 56

Communication
3.3 S7 connections
Assigning connections in the program
In S7 basic communication, and in open Industrial Ethernet communication with TCP/IP, the
user program establishes the connection. The CPU operating system initiates the
connection. S7 basic communication uses the corresponding S7 connections. The open
IE communication does not use any S7 connections. The maximum number of eight
connections also applies to this type of communication.
Using connections for commissioning, testing and diagnostics
An active online function on the engineering station (PG/PC with STEP 7) occupies
S7 connections for PG communication:
• An S7 connection resource for PG communication which was reserved in your CPU
• If all reserved S7 connection resources for PG communication are allocated, the
Allocating connection resources to OCMS services
An online function of the OCM station (OP/TP/... with
resources for OP communication:
• An S7 connection resource for OP communication you have reserved in your CPU
• If all reserved S7 connection resources for OP communication are allocated, the
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 CPU's 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.
See also
Open communication via Industrial Ethernet (Page 3-24)
3-28
hardware configuration is assigned to the engineering station, that is, it only needs to be
allocated.
operating system automatically assigns a free S7 connection resource which has not yet
been reserved. If no more connection resources are available, the engineering station
cannot go online to the CPU.
hardware configuration is therefore assigned to the OCM station engineering station, that
is, it only needs to be allocated.
operating system automatically assigns a free S7 connection resource which has not yet
been reserved. If no more connection resources are available, the OCM station cannot go
online to the CPU.
ProTool
) allocates S7 connection
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05