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. Operator interfaces that allow monitoring and issuing process commands, such as controller set point changes, are handled through the SCADA supervisory computer system. However, real-time control logic or controller calculations are performed using networked modules that connect to other peripheral devices, such as programmable logic controllers and discrete PID controllers that connect 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 and allow access via standard automation protocols. In practice, large SCADA systems have become very similar to distributed control systems in function, but using multiple means of interfacing with the plant. SCADA systems are vulnerable to cyber warfare or cyberterrorism attacks.[3]​.

SCADA software works at a supervisory level as control actions are performed automatically using RTU or PLC. SCADA control functions are typically restricted to basic or supervisory intervention. The RTU or PLC directly controls a feedback control circuit, but the SCADA software controls the overall performance of the circuit. For example, a PLC can control the flow of cooling water through part of an industrial process to a set point level, but the SCADA system software will allow operators to change the set points for the flow. The SCADA also allows alarm conditions, such as loss of flow or high temperature, to be displayed and recorded.

PLCs can range from small modular devices with dozens of inputs and outputs (I/O) in a housing integral with the processor, to large rack-mounted modular devices with a count of thousands of I/Os, and which are often networked to other PLC and SCADA systems. They can be designed for multiple digital and analog input and output arrangements, extended temperature ranges, immunity to electrical noise (Noise (Physics)), and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory.

The process control of large industrial plants has evolved through many stages. Initially, control was from local panels to the process plant. However, this required staff to attend to these scattered panels, and there was no overview of the process. The next logical development was the transmission of all plant measurements to a permanently staffed central control room. Controllers were often behind control room panels, and all manual and automatic control outputs were transmitted individually to the plant in the form of pneumatic or electrical signals. Effectively, this was the centralization of all localized panels, with the benefits of reduced staffing requirements and consolidated overview of the process.

However, although it provided a central control approach, this arrangement was inflexible, as each control loop had its own controller hardware, so changes to the system required reconfiguration of signals using new plumbing or rewiring. It also required continuous movement of the operator within a large control room in order to monitor the entire process. With the advent of electronic processors, high-speed electronic signaling networks, and electronic graphic displays, it became possible to replace these discrete controllers with computer-based algorithms, housed in a network of input/output racks with their own control processors. These could be distributed around the plant and would communicate with graphic displays in the control room. The concept of distributed control was realized.

The introduction of distributed control enabled flexible interconnection and reconfiguration of plant controls, such as cascading loops and interlocks, and interconnection with other production computing systems. It enabled sophisticated alarm handling, introduced automatic event logging, eliminated the need for physical records such as chart recorders, allowed control racks to be networked and therefore located locally for the plant to reduce wiring runs, and provided high-level overview information on plant status and production levels. For large control systems, the general trade name distributed control system (DCS) was coined to refer to proprietary modular systems from many manufacturers that integrated high-speed networks and a complete set of displays and control racks.

While the DCS was adapted to meet the needs of large industrial continuous processes, in industries where combinatorial and sequential logic was the primary requirement, the PLC (programmable logic controller) evolved from the need to replace timer racks and timers used for events. driven control Older controls were difficult to reconfigure and find faults, and PLC control allowed signals to be networked to a central control area with electronic displays. PLCs were first developed for the automotive industry on vehicle production lines, where sequential logic was becoming very complex. It was soon adopted in a number of other event-driven applications as varied as printing presses and water treatment plants.

The history of SCADA is in distribution applications, such as power, natural gas, and water pipelines, where there is a need to collect remote data over potentially unreliable or intermittent low-bandwidth links. SCADA systems use open loop control with sites that are widely separated geographically. A SCADA system uses RTUs (remote terminal units, also known as remote telemetry units) to send monitoring data to a control center. Most RTU systems have always had a limited ability to handle local controls while the master station is unavailable. However, over the years, RTU systems have grown increasingly capable of handling local controls.

The boundaries between DCS and SCADA/PLC systems are blurring as time goes on.[4]​ The technical boundaries that drove the designs of these various systems are no longer as problematic. Many PLC platforms can now function quite well as a small DCS, using remote I/O, and are reliable enough for some SCADA systems to actually manage closed loop control over long distances. With the increasing speed of today's processors, many DCS products have a full line of PLC-like subsystems that were not offered when they were initially developed.

In 1993, with the release of IEC-1131, which later became IEC-61131-3, the industry moved toward greater code standardization, hardware-independent, reusable control software. For the first time, object-oriented programming (OOP) became possible within industrial control systems. This led to the development of both programmable automation controllers (PAC) and industrial PCs (IPC). These are platforms programmed in the 5 standardized IEC languages ​​(ladder logic, structured text, function block, instruction list and sequential function table). They can also be programmed in modern high-level languages ​​such as C or C++. In addition, they accept models developed in analytical tools such as MATLAB and Simulink. Unlike traditional PLCs, which use proprietary operating systems, IPCs use Windows IoT. IPCs have the advantage of powerful multi-core processors with much lower hardware costs than traditional PLCs and adapt well to multiple form factors such as DIN rail mounting, combined with a touch screen as a "panel-pc" or as an embedded PC. New hardware platforms and technology have contributed significantly to the evolution of DCS and SCADA systems, further blurring boundaries and changing definitions.

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. Operator interfaces that allow monitoring and issuing process commands, such as controller set point changes, are handled through the SCADA supervisory computer system. However, real-time control logic or controller calculations are performed using networked modules that connect to other peripheral devices, such as programmable logic controllers and discrete PID controllers that connect 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 and allow access via standard automation protocols. In practice, large SCADA systems have become very similar to distributed control systems in function, but using multiple means of interfacing with the plant. SCADA systems are vulnerable to cyber warfare or cyberterrorism attacks.[3]​.

SCADA software works at a supervisory level as control actions are performed automatically using RTU or PLC. SCADA control functions are typically restricted to basic or supervisory intervention. The RTU or PLC directly controls a feedback control circuit, but the SCADA software controls the overall performance of the circuit. For example, a PLC can control the flow of cooling water through part of an industrial process to a set point level, but the SCADA system software will allow operators to change the set points for the flow. The SCADA also allows alarm conditions, such as loss of flow or high temperature, to be displayed and recorded.

PLCs can range from small modular devices with dozens of inputs and outputs (I/O) in a housing integral with the processor, to large rack-mounted modular devices with a count of thousands of I/Os, and which are often networked to other PLC and SCADA systems. They can be designed for multiple digital and analog input and output arrangements, extended temperature ranges, immunity to electrical noise (Noise (Physics)), and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory.

The process control of large industrial plants has evolved through many stages. Initially, control was from local panels to the process plant. However, this required staff to attend to these scattered panels, and there was no overview of the process. The next logical development was the transmission of all plant measurements to a permanently staffed central control room. Controllers were often behind control room panels, and all manual and automatic control outputs were transmitted individually to the plant in the form of pneumatic or electrical signals. Effectively, this was the centralization of all localized panels, with the benefits of reduced staffing requirements and consolidated overview of the process.

However, although it provided a central control approach, this arrangement was inflexible, as each control loop had its own controller hardware, so changes to the system required reconfiguration of signals using new plumbing or rewiring. It also required continuous movement of the operator within a large control room in order to monitor the entire process. With the advent of electronic processors, high-speed electronic signaling networks, and electronic graphic displays, it became possible to replace these discrete controllers with computer-based algorithms, housed in a network of input/output racks with their own control processors. These could be distributed around the plant and would communicate with graphic displays in the control room. The concept of distributed control was realized.

The introduction of distributed control enabled flexible interconnection and reconfiguration of plant controls, such as cascading loops and interlocks, and interconnection with other production computing systems. It enabled sophisticated alarm handling, introduced automatic event logging, eliminated the need for physical records such as chart recorders, allowed control racks to be networked and therefore located locally for the plant to reduce wiring runs, and provided high-level overview information on plant status and production levels. For large control systems, the general trade name distributed control system (DCS) was coined to refer to proprietary modular systems from many manufacturers that integrated high-speed networks and a complete set of displays and control racks.

While the DCS was adapted to meet the needs of large industrial continuous processes, in industries where combinatorial and sequential logic was the primary requirement, the PLC (programmable logic controller) evolved from the need to replace timer racks and timers used for events. driven control Older controls were difficult to reconfigure and find faults, and PLC control allowed signals to be networked to a central control area with electronic displays. PLCs were first developed for the automotive industry on vehicle production lines, where sequential logic was becoming very complex. It was soon adopted in a number of other event-driven applications as varied as printing presses and water treatment plants.

The history of SCADA is in distribution applications, such as power, natural gas, and water pipelines, where there is a need to collect remote data over potentially unreliable or intermittent low-bandwidth links. SCADA systems use open loop control with sites that are widely separated geographically. A SCADA system uses RTUs (remote terminal units, also known as remote telemetry units) to send monitoring data to a control center. Most RTU systems have always had a limited ability to handle local controls while the master station is unavailable. However, over the years, RTU systems have grown increasingly capable of handling local controls.

The boundaries between DCS and SCADA/PLC systems are blurring as time goes on.[4]​ The technical boundaries that drove the designs of these various systems are no longer as problematic. Many PLC platforms can now function quite well as a small DCS, using remote I/O, and are reliable enough for some SCADA systems to actually manage closed loop control over long distances. With the increasing speed of today's processors, many DCS products have a full line of PLC-like subsystems that were not offered when they were initially developed.

In 1993, with the release of IEC-1131, which later became IEC-61131-3, the industry moved toward greater code standardization, hardware-independent, reusable control software. For the first time, object-oriented programming (OOP) became possible within industrial control systems. This led to the development of both programmable automation controllers (PAC) and industrial PCs (IPC). These are platforms programmed in the 5 standardized IEC languages ​​(ladder logic, structured text, function block, instruction list and sequential function table). They can also be programmed in modern high-level languages ​​such as C or C++. In addition, they accept models developed in analytical tools such as MATLAB and Simulink. Unlike traditional PLCs, which use proprietary operating systems, IPCs use Windows IoT. IPCs have the advantage of powerful multi-core processors with much lower hardware costs than traditional PLCs and adapt well to multiple form factors such as DIN rail mounting, combined with a touch screen as a "panel-pc" or as an embedded PC. New hardware platforms and technology have contributed significantly to the evolution of DCS and SCADA systems, further blurring boundaries and changing definitions.