To ease further discussion of OPC, a model of OPC communication is introduced, see Fig. 1.
Fig. 1 shows a general model for how an application (1), with an integrated OPC client (2), can connect to a PLC (8). There are three different communication parts in this model: the OPC interface (3), i.e., COM/DCOM, the communication interface (5) between the OPC server (4) and the gateway (6), and the gateway interface (7) to the PLC.
There might be other models, where, e.g., the OPC server has an integrated gateway, but the principle is still the same. Another scenario is an integrated OPC server in the PLC, where (6) and (7) might be unnecessary. In general, the model is valid for a standard OPC connection.