QUESTION:
What do I have to consider when connecting a PC to a S7-300 and  S7-400?

ANSWER:
In figure 1 you see a configured connection from an OPC-Server to a SIMATIC 400 and an unconfigured connection from a OPC-Server to the SIMATIC 300. 

Figure 1  Connection configuration in NETPRO

Figure 2  Configured connection (S7 400)  

Figure 3:  Unconfigured connection (S7-300)

In figure 2 you can see that a bi-directional connection has been configured for the SIMATIC 400.

In figure 3 you can see that a uni-directional connection has been configured for the SIMATIC 300.

After the connection has been established the configuration has to be transferred to the PC station.

Detailed informationen about this topic can be found on the SIMATIC NET CD under "Advanced PC Configuration" 

ATTENTION:
The configured connection has to be transferred also onto the S7-400 station.

Explanation:
When configuring a connection to a S7-400 in NETPRO, the project has to be transferred to both stations, here the "PC_Station" and the "SIMATIC 400", with "Load to station". This is called a configured connection. The S7 connections that have been defined on the server as well as on the client side - e.g. from the PC to a AS S7-400 station - are called also bi-directional connections.

When configuring a connection to a S7-300 in NETPRO, the project has to be transferred only to the PC station. This is an unconfigured connection. S7 connections that are defined only on the client side - e.g. from a PC to an AS S7-300 station - are also called uni-directional.