What is SCADA?: The Basics you Need to Know

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05 Sep, 2021

What is SCADA?: The Basics you Need to Know

How SCADA systems enable Integrated and Effective Industrial Control Systems

SCADA stands for Supervisory Control and Data Acquisition. SCADA systems capture, analyze, and control equipment and its real-time data in order to ensure critical or time-sensitive materials and events are handled effectively. 

For over four decades the operation of different types of industrial plants (e.g., manufacturing plants, nuclear power plants, oil pipelines) depends on the deployment and use of industrial control systems. The latter comprises various systems and instrumentation, which are used to control industrial processes. 

Once upon a time, industrial control relied on simple automation devices such as CNC (Computerized Numerical Control), PLC (Programmable Logic Controller), and PID (Proportional, Integral, Derivative) control devices.  For instance, CNC machines comprise computerized manufacturing processes that leverage pre-programmed software code to control the movement of production equipment. As another example, PLCs are industrial devices that are programmed to control electro-mechanical processes in industrial environments. Likewise, PID controllers enable accurate control of industrial processes based on a continuous variation of output within a control loop. As such, they can remove oscillation and increase process efficiency. 

Nevertheless, PLC, CNC, and PID control systems offer very poor transparency in their operation, as they are typically programmed in low-level and difficult-to-understand assembly languages. Furthermore, they provide limited flexibility and functionality, which makes them inappropriate for implementing sophisticated industrial control scenarios. To alleviate these limitations, SCADA systems have emerged and are widely used for controlling industrial equipment and processes.

What is SCADA? An Introduction to SCADA Systems

SCADA systems were an evolutionary step in the development of non-trivial industrial control systems. Rather than controlling a single field device based on a given protocol, SCADA systems support multiple protocols, which provides increased versatility and scalability in monitoring and controlling field devices [1].  Specifically, SCADA enables data capture from remote devices and facilitates the transferring of data to a central location. Moreover, it supports the visualization of processes through proper Human Machine Interfaces (HMI), as well as the monitoring and control of the plant through SCADA hosts.

To provide these functionalities, SCADA implements sophisticated control system architectures, which comprise computers, networked data communication infrastructures, peripheral devices (e.g., PLC and PID micro-controllers), as well as HMIs. Leveraging these components, SCADA systems provide management information about industrial control processes such as information about scheduled maintenance procedures and diagnostic information about sensors and machines. Furthermore, SCADA provides real-time insights on industrial operations towards safeguarding their efficiency, while detecting and mitigating potential problems in a timely fashion. For instance, SCADA systems issue alerts whenever abnormalities in some industrial processes are observed. It also facilitates the initiation of restoration actions upon the issue of the alerts.

SCADA based control architectures are very popular and widely used in different industrial sectors. For example, SCADA is deployed for industrial control in settings like water management, electric generation, energy transmission and distribution, oil and gas production, and manufacturing plants. SCADA’s popularity lies in its ability to offer increased reliability, cost efficiency, optimal utilization of assets and resources, as well as improved worker safety. 

The Main Components of a SCADA System

  • Supervisory Computers: A SCADA comprises multiple computers, including one or more server computers that run SCADA software. SCADA software manages the flow of information between software and industrial equipment. A SCADA server resides at the heart of the control architecture usually in star network topology. The servers communicate with the PLC and RTU devices of the SCADA network. In this way, they collect information about the individual processes controlled by the PLCs and RTUs, which allows them to build a consolidated picture of the overall industrial process. Likewise, the SCADA servers update the historical databases of the control architecture with information derived by RTUs and PLCs.  In small-scale deployments, a single SCADA workstation performs the work of both the server and of the user host i.e., the HMI of the deployment. However, more sophisticated deployments deploy more than one workstation.

  • Field Devices: A SCADA control architecture comprises various field devices that interface to the physical world. Prominent examples of field devices include wireless sensors, reservoir level meters, water flow meters, and multi-variable transmitters. These devices transmit their data to RTUs, which undertake to convey the data to the SCADA supervisory servers.

  • Remote Terminal Units (RTU): RTUs are electronic devices that interconnect physical world objects (e.g., sensors, flow meters) to automation systems like SCADA and other Distributed Control Systems (DCS). As their name indicates, RTUs can be deployed in remote locations i.e., locations that differ from where the SCADA servers are deployed. They are controlled by microprocessors, which enable them to capture and transmit telemetry data to the SCADA supervisory system i.e. the SCADA server. In the scope of a SCADA control architecture, RTUs provide the means for controlling physical world objects. State-of-the-art RTUs are industrial-grade devices, which operate in harsh industrial environments and offer backup power options. Moreover, modern RTUs support the implementation and configuration of their control logic by means of popular programming languages like Basic, Visual Basic, and C++.

  • Programmable Logic Controllers (PLC): PLCs are solid-state devices that control output devices in a programmable fashion. In this direction, PLCs comprise a microcontroller and operate based on information from input devices. PLCs process the input data and trigger outputs based on parameters that are pre-programmed in the microcontroller.  Similar to RTUs, PLCs connect to field devices in order to control them. However, they are more configurable, programmable, economical, and flexible than RTUs.

  • Human-machine interface (HMI): Every SCADA system comprises an HMI, which is hosted on a computer. The latter is called “operator station”, “work station” or “control client”. The role of the HMI is to provide a graphical display of the status of the industrial processes that are controlled by the SCADA system. To this end, it processes data towards monitoring, analyzing, and visualizing control processes. In this direction, the HMI facilitates the engagement of automation engineers based on interactive interfaces. In most cases, HMIs comprise a “historian” software service, which is destined to collect and aggregate past data, events, and alarms in a database. This historian service facilitates the extraction and visualization of trends in the HMI. Specifically, the service comprises a client that queries the historian and visualizes data in the HMI.

  • Communication infrastructure: SCADA control architectures are empowered by proper communication infrastructures, which facilitate the exchange of data and information between the above-listed components. SCADA communication infrastructures comprise different networked elements and technologies, including radio networks, modems, satellites, routers, and switches to support complex networked topologies at various scales. In cases where the SCADA components reside in a limited area (e.g., an industrial plant) the communication infrastructure can be structured as a Local Area Network (LAN). Overall, SCADA control architectures can be supported by various popular networked topologies such as the bus, star, and ring topologies.

Leveraging the above listed elements, a SCADA control architecture can be structured as a layered system. The latter comprises the following levels:

  • Level 0, which is the level where field devices such as sensors (e.g., flow sensors, temperature sensors) and actuators (e.g., control valves) reside.

  • Level 1, which comprises industrialized input/output (I/O) modules that interface to the field devices, including PLCs and RTUs.

  • Level 2, which consists of supervisory computers such as SCADA servers. These computers collect and aggregate information from processor nodes while offering interfaces to the system’s operators. 

  • Level 3, which deals with higher level production control. Specifically, it monitors production assets and KPIs (Key Performance Indicators).

  • Level 4, which offers high level production scheduling functionalities, leveraging information and capabilities from the lower layers.

PLC and SCADA: Knowing the Difference

SCADA systems are commonly used in conjunction with PLCs, which sometimes creates confusion regarding their exact operation and scope. While they are both widely used in industrial automation solutions, they have many differences in their nature and functionality. PLC is a programmable hardware element, while SCADA is primarily comprised of middleware and software modules that interconnect different pieces of hardware, including PLCs. Likewise, while PLCs are used for controlling industrial elements (e.g., field devices and processes) locally, SCADA has a much wider scope: It is an integrated control system for an industrial plant, which ends up comprising many hardware, software, and middleware elements. Using a SCADA system, industrial engineers can gain insights into a richer and wider set of industrial processes, beyond the parts or devices controlled by a PLC. This is also the reason why a SCADA control architecture comprises several PLC components i.e., in most cases PLC and SCADA co-exist in the scope of an industrial control platform.  The SCADA communication infrastructure provides the means for interfacing PLC with SCADA through a proper communication channel. Overall, SCADA offers more integrated, more versatile, and richer automation and control functionalities than a PLC. In most cases, PLCs are deployed as a subset of a SCADA control architecture [3].

SCADA in the Industry 4.0 and Industrial Internet of Things (IIoT) Era

The advent of SCADA systems has signaled a significant improvement in industrial control functionalities. This is because SCADA eases the monitoring and integration of several industrial control processes, based on a variety of field devices. It also helps understand the context of the industrial processes, based on HMIs that are much more user-friendly than the low-level programming interfaces of PLC, CNC, and PID devices. 

In recent years, the advent of the fourth industrial revolution (Industry 4.0) and of the Industrial Internet of Things (IIoT) holds the promise to increase the decentralization, resilience, and functional sophistication of industrial control systems. Specifically, IIoT enables decentralized and virtualized control architectures, which are much more flexible than centralized SCADA systems. Instead of collecting and aggregating signals in a single database (e.g., a historian), IIoT-based control systems will be able to handle large volumes of highly distributed data in a virtualized fashion. This will eliminate single points of failure while easing the task of managing BigData and applying advanced analytics (e.g., Machine Learning) for higher intelligence, automation, and optimization. Nevertheless, IIoT is not expected to replace SCADA systems in the near future. Rather SCADA will act as primary data sources of IIoT systems while providing the first level of integrated industrial control functionalities. In the foreseeable future, industrial organizations will opt for hybrid decentralized architectures that will include centralized elements like SCADA and DCS as data sources and points of local intelligence in industrial control [4]. SCADA systems are popular and here to stay.


  1. D. Pliatsios, P. Sarigiannidis, T. Lagkas and A. G. Sarigiannidis, "A Survey on SCADA Systems: Secure Protocols, Incidents, Threats and Tactics," in IEEE Communications Surveys & Tutorials, vol. 22, no. 3, pp. 1942-1976, thirdquarter 2020, doi: 10.1109/COMST.2020.2987688.
  2. Jingcheng Gao, Jing Liu, Bharat Rajan, Rahul Nori, BoFu, Yang Xiao, Wei Liang and C. L. Philip Chen, “SCADA communication and security issues”, Security and Communication Networks 2014; 7:175– 194, https://onlinelibrary.wiley.com/doi/epdf/10.1002/sec.698
  3. M. Endi, Y. Z. Elhalwagy and A. hashad, "Three-layer PLC/SCADA system Architecture in process automation and data monitoring," 2010 The 2nd International Conference on Computer and Automation Engineering (ICCAE), 2010, pp. 774-779, doi: 10.1109/ICCAE.2010.5451799.
  4. R. Khan, K. McLaughlin, B. Kang, D. Laverty and S. Sezer, "A Seamless Cloud Migration Approach to Secure Distributed Legacy Industrial SCADA Systems," 2020 IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT), 2020, pp. 1-5, doi: 10.1109/ISGT45199.2020.9087760.

More by John Soldatos

John Soldatos holds a PhD in Electrical & Computer Engineering from the National Technical University of Athens (2000) and is currently Honorary Research Fellow at the University of Glasgow, UK (2014-present). He was Associate Professor and Head of the Internet of Things (IoT) Group at the Athens In...

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