Siemens SIMATIC NET S7-300 Manual page 105

Tasks of the Ethernet CP
When handling the data transfer on a connection, the Ethernet CP has the
following tasks:
S When receiving
Receiving data from Ethernet and transferring it to the user data area on the
CPU.
S When sending
Taking data from the user data area on the CPU and sending the data via
Ethernet.
The connection is established automatically as soon as the partner is obtainable.
For a free UDP connection, the following functions are also necessary in addition
to those above:
S When receiving
Entry of the sender of the message in the job header.
S When sending
Evaluation of the job header and addressing the partner
Requirements for Configuring Connections
The Ethernet CP was entered in the hardware configuration and connected to the
subnet.
As a bus node, the Ethernet CP has an address.
Notice
All stations not in the STEP 7 project must be configured with substitute objects
(for example as "SIMATIC S5" or "Other stations").
or
Use the "unspecified" partner type when you create the connection.
S7-CPs for Industrial Ethernet Configuring and Commissioning
Release 01/2007
C79000-G8976-C182-07
5 Configuring Communication Connections
A -105