In this scenario, the edge is not directly connected to the data source but to its OPC Classic server through an OPC Classic client:

This means that the edge must implement and incorporate the OPC Classic client interface. The data source could be one of those mentioned previously—a PLC, a DCS, a SCADA system, or a Historian. The OPC abstracts the data source, which is why, in the preceding diagram, the OPC server has a dotted link with each of the potential data sources. Obviously, the OPC server must be specific to a particular data source, but, from the perspective of the client and the edge, this does not matter. Whichever data source is connected, the OPC server will expose its data through an OPC Classic interface to which the edge can connect through its OPC Classic client interface. This is the OPC-DA client interface for the PLC, DCS, and SCADA systems, and the OPC-HDA interface for the Historian system. According to the underlying data source abstracted by the OPC server, we will have the related advantages and disadvantages that were discussed and analyzed in the previous section.