Supervisory Control and Data Acquisition (SCADA) systems are used to control and automate industrial processes. SCADA systems includes:
Supervisory computers: the servers used to manage the process gathering data on the process and communicating with field devices (PLC/RTU). In smaller deployments HMI is embedded in a single computer, in larger deploy HMI is installed into a dedicated computer.
Programmable Logic Controllers (PLC): digital computers used mainly for automating industrial processes. They are used to continuously monitor sensors (input) and make decisions controlling devices (output).
Remote Terminal Units (RTU): nowadays RTUs and PLCs functionalities overlap with each other. RTUs are usually preferred for wider geographical telemetry whereas PLCs are better with local controls. Are electronic devices located at key measurement and control points. Remote field devices that collect data from sensors and transmit it to the SCADA system. Originally hardwired devices with limited capabilities and one proprietary communication protocol.
Communication networks: the network connecting all SCADA components (Ethernet, Serial, telephones, radio, cellular...). Network failures do not necessarily impact negatively on the plant process. Both RTU's and PLC's should be designed to operate autonomously, using the last instruction given from the supervisory system. Latest systems are digital networks designed to transfer data using TCP/IP and wireless including cellular and satellite.
Human Machine Interface (HMI): displays a digitalized representation of the plant. Operators can interact with the plant issuing commands using mouse, keyboards or touch screens. Operators can make supervisory decisions adjusting or overriding the normal plant behaviour. ⚠️ If an attacker can gain access to HMI, he can control the system!
To summarize, SCADA systems are geographically dispersed, with communications occurring over long distances and at low speeds, and with control delegated to the supervisory level.