A Programmable Logic Controller (PLC) is a microprocessor running special software. That software has communications elements, some of which are accessible to an internal database.
Supervisory Control and Data Acquisition software (SCADA) typically runs on a “standard” PC or Server. It, to, has communications elements, some of which are accessible to its own internal database.
When you wish to connect a PLC to SCADA, you first must establish the physical communication connection. Industrial Ethernet, for example. Industial Token Ring networks like ControlNet as another example. In either instance, the communications or network port of the PC will connect to wiring (usually) which connects to the PLC.
The next step is to assure that both systems can now communicate with each other, using what ever protocol you have selected for this purpose.
Finally, you will need to configure the SCADA database to gather the information across the network from the PLC database. This is typically done by having names or addresses in the PLC map to typically similar names within the SCADA.
Once that is all done, information may flow back and forth between PLC and SCADA, and various control aspects may be programmed into the SCADA to execute on the PLC, and similarly information collected by the PLC updates the SCADA, typically graphic elements.
Those are the basic steps. The details vary widely from manufacturer to manufacturer, and in many instances some or all of the software involved may be licensed and “closed source” for both revenue and security purposes. Consult your PLC and SCADA manufacturer for their specific details and recommendations.
Author - Ned Boff