Supervisory Control and Data Acquisition (SCADA) is the process by which automation applications collect data and use it for supervisory control.
People sometimes use SCADA to refer to the combination of an HMI and a PLC. The HMI acquires data from the PLC and enables an operator to make supervisory control decisions. The operator can change setpoints, which enables them to handle supervisory control. This type of application is marked by a relatively small number of PLCs, with a relatively large number of I/O points, which are always connected to the SCADA host in a small geographic area (such as a plant).
Other people refer to SCADA for remote control with RTUs. This scenario is similar to the HMI/PLC combination, except that RTUs are used instead of PLCs, and telemetry is typically used for a central SCADA host to collect data. In this application of the technology, the RTUs are often disconnected from their host and have to provide autonomous control. SCADA is often used by utility corporations for their transmission and distribution requirements, by Oil & Gas producers for the extraction of oil and gas, by pipeline companies to manage long pipelines, etc. This type of application is marked by a relatively large number of RTUs, with a relatively small number of I/O points, which are typically disconnected from the SCADA host, in a large geographic area (such as city, state, or country).
OPC can be used for all SCADA applications. Special care must be taken to differentiate between OPC products that are aimed at a plant versus those aimed at telemetry SCADA installations.
See also: HMI, PLC, RTU