SCADA is the abbreviation for Supervisory Control And Data Acquisition. It generally refers to an industrial control system which is meant to function across a wide area with an autonomous Remote Terminal Unit (RTU). The precise definition of SCADA has been muddied somewhat by newer telecommunications technology, enabling reliable, low latency, high speed communications over wide areas, and a tendency by popular media to mistakenly refer to all Industrial Control Systems as SCADA. Despite this confusion, a SCADA system is expected to have open loop controls (meaning that a human operator watches near real time data and issues commands). By comparison, a Distributed control system (DCS) is expected to have closed loop controls (meaning that real-time loop data is applied directly to an industrial controller without human intervention). These differences are primarily design philosophies, not mandates of definition.
The supervisory control system is a system that sends commands to a real-time control system to control a process that is external to the SCADA system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling). This implies that the system coordinates, but does not control processes in real time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time constants of the process. The process can be industrial, infrastructure or facility based as described below:
* Industrial processes include those of manufacturing, production, power generation, fabrication, and refining, and may run in continuous, batch, repetitive, or discrete modes.
* Infrastructure processes may be public or private, and include water treatment and distribution, wastewater collection and treatment, oil and gas pipelines, electrical power transmission and distribution, and large communication systems.
* Facility processes occur both in public facilities and private ones, including buildings, airports, ships, and space stations. They monitor and control HVAC, access, and energy consumption.
SCADA systems, a branch of instrumentation engineering, include input-output signal hardware, controllers, human-machine interfacing ("HMI"), networks, communications, databases, and software.
The term SCADA usually refers to centralized systems which monitor and control entire sites, or complexes of systems spread out over large areas (on the scale of kilometers or miles). Most site control is performed automatically by remote terminal units ("RTUs") or by programmable logic controllers ("PLCs"). Host control functions are usually restricted to basic site overriding or supervisory level intervention. For example, a PLC may control the flow of cooling water through part of an industrial process, but the SCADA system may allow operators to change the set points for the flow, and enable alarm conditions, such as loss of flow and high temperature, to be displayed and recorded. The feedback control loop passes through the RTU or PLC, while the SCADA system monitors the overall performance of the loop.
Data acquisition begins at the RTU or PLC level and includes meter readings and equipment status reports that are communicated to SCADA as required. Data is then compiled and formatted in such a way that a control room operator using the HMI can make supervisory decisions to adjust or override normal RTU (PLC) controls. Data may also be fed to a Historian, often built on a commodity Database Management System, to allow trending and other analytical auditing.
SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point represents an actual input or output within the system, while a soft point results from logic and math operations applied to other points. (Most implementations conceptually remove the distinction by making every property a "soft" point expression, which may, in the simplest case, equal a single hard point.) Points are normally stored as value-timestamp pairs: a value, and the timestamp when it was recorded or calculated. A series of value-timestamp pairs gives the history of that point. It's also common to store additional metadata with tags, such as the path to a field device or PLC register, design time comments, and alarm information.
