Supervisory control and data acquisition (SCADA) systems are large-scale, industrial control and measurement systems that consist of a central host, one or more remote units, a communications network, and specialized software. With SCADA systems, the central monitoring system (CMS) or master terminal unit (MTU) receives inputs from remote terminal units (RTU) that are connected to field instrumentation such as process ovens. When polled by the SCADA master, the RTUs transmit data to the central host for processing. SCADA software then logs alarms and displays the data graphically.

Today, many supervisory control and data acquisition (SCADA) systems are designed to run on personal computers (PCs) that use popular operating systems such as Microsoft Windows (Microsoft) and UNIX (The Open Group). Selecting supervisory control and data acquisition (SCADA) systems requires an analysis of remote terminal units (RTU), devices which some suppliers call digital communication units (DCS).

There are two main types of remote terminals: single-board and modular. Single-board RTUs are compact, ruggedized products which locate all of the input/output (I/O) modules on a single printed circuit board (PCB). Modular RTUs have a separate, central processing unit (CPU) and a backplane for connecting other I/O modules. For both types of remote terminals, specifications include power consumption, physical size, operating temperature, relative humidity, maximum addressability, and communication type. Some supervisory control and data acquisition (SCADA) systems work with RTUs that perform basic alarm processing, or initiate reporting to the SCADA master. Supervisory control and data acquisition (SCADA) systems use a central monitoring system to perform a variety of tasks. These functions include real-time data collection, automatic reporting, sequential control, password protection diagnostics, and event-driven processing. SCADA software with screen blanking, help files, popup windows, and online file utilities is also available.

Most supervisory control and data acquisition (SCADA) systems support RTU communications via dial-up, Ethernet, radio frequency (RF), satellite, or serial communications. Ethernet is a local area network (LAN) protocol that uses a bus or star typology and supports data transfer rates of 10 Mbps. To handle simultaneous demands, Ethernet uses carrier sense multiple access / collision detection (CSMA/CD) to monitor network traffic. Serial communications for SCADA systems include RS232, RS422, and RS485.