Supervisory control and data acquisition (SCADA) is a control system architecture that uses computers, networked data communications and graphical user interfaces for high-level process supervisory management. The operator interfaces, which enable monitoring and the issuing of process commands such as controller setpoint changes, are handled through the SCADA supervisory computer system. However, the real-time control logic or controller calculations are typically performed by networked modules which connect to other peripheral devices such as programmable logic controllers (PLCs) and/or discrete PID controllers which interface to the process plant or machinery.
The SCADA concept was developed as a universal means of remote access to a variety of local control modules, which could be from different manufacturers allowing access through standard automation protocols. In practice, large SCADA systems have grown to become very similar to distributed control systems in function but using multiple means of interfacing with the plant. They can control large-scale processes that can include multiple sites and work over large distances.
The SCADA software operates on a supervisory level as control actions are performed automatically by PLCs. SCADA control functions are usually restricted to basic overriding or supervisory level intervention. A feedback control loop is directly controlled by the PLC, but the SCADA software monitors the overall performance of the loop. For example, a PLC may control the flow of cooling water through part of an industrial process to a setpoint level, but the SCADA system software will allow operators to change the setpoints for the flow. The SCADA also enables alarm conditions, such as loss of flow or high temperature, to be displayed and recorded.