The term SCADA usually refers to centralized systems which monitor and control entire sites, or complexes of systems spread out over large areas (anything from an industrial plant to a nation). Most control actions are performed automatically by PLCs. Host control functions are usually restricted to basic overriding or supervisory level intervention.
A SCADA system usually consists of the following subsystems:
A human–machine interface or HMI is the apparatus which presents process data to a human operator, and through this, the human operator monitors and controls the process.
A supervisory (computer) system, gathering (acquiring) data on the process and sending commands (control) to the process.
Remote terminal units (RTUs) connecting to sensors in the process, converting sensor signals to digital data and sending digital data to the supervisory system.
Programmable logic controller (PLCs) used as field devices because they are more economical, versatile, flexible, and configurable than special-purpose RTUs.
Communication infrastructure connecting the supervisory system to the remote terminal units. Various process and analytical instrumentation
