Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 User Manual

Page 56

Communication

3.3 S7 connections

CPU 31xC and CPU 31x, Technical data

3-28

Manual, Edition 08/2004, A5E00105475-05

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

hardware configuration is assigned to the engineering station, that is, it only needs to be

allocated.

•

If all reserved S7 connection resources for PG communication are allocated, the

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.

Allocating connection resources to OCMS services

An online function of the OCM station (OP/TP/... with

ProTool) allocates S7 connection

resources for OP communication:
•

An S7 connection resource for OP communication you have reserved in your CPU

hardware configuration is therefore assigned to the OCM station engineering station, that

is, it only needs to be allocated.

•

If all reserved S7 connection resources for OP communication are allocated, the

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.

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.