The S7online-interface represents the OSI layers 1 to 4 for all applications inside the Simatic universe. If a Simatic application communicates to a PLC, the data goes always through the S7online-interface. The functions of the S7online-interface are accessible through the program library s7onlinx.dll inside the windows system directory.

The S7online-interface passes the data from the application to the underlying transport layer, like TCP/IP, MPI or Profibus. The transport layer which S7online is using, is configured in the dialog “Set PG/PC interface” from the Simatic application or the Windows control panel. Also the communication to Plcsim runs through this interface. The data running through this interface are already S7-communication. Thus the job of NetToPLCsim is to represent the IP/IsoOnTcp transport layer, and pass the payload into the S7online-interface and back. The main problem with the S7online-interface is that it’s not official documented, which was the main problem using this interface with NetToPLCsim.

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