What is a SCADA System, and How Does It Work?

Serhii Orlivskyi23.04.202523.04.2025

Table of Contents

Internet of Things (IoT) projects often generate large volumes of data that organizations must process in real or near real-time. As your projects grow, it becomes increasingly difficult to manage your data efficiently. To solve this problem for industrial IoT use cases like controlling manufacturing processes and critical infrastructure, you should consider using a Supervisory Control and Data Acquisition (SCADA) system.

SCADA systems enable organizations to control processes remotely and monitor and process data in real time. They can even interact with various devices such as sensors or motors. At a high level, SCADA systems provide observability over the entire system. They enhance efficiency by utilizing data for smarter decision-making and alert users of potential system issues.

This article explores SCADA systems, how they work, the different types, and the key operational principles behind them.

The definition of SCADA

So, what is SCADA? As mentioned above, SCADA stands for Supervisory Control and Data Acquisition. It is a comprehensive system that uses hardware and software. Together, they facilitate process observability, automation, and control by capturing real-time operational data.

SCADA was first used in the 1960s and has since become a key component in many industrial plants and production facilities. SCADA systems connect on-site or remote servers to sensors that monitor equipment like motors and pumps. System operators can leverage captured data to make more informed and intelligent decisions.

Now that you understand SCADA systems and their function, let’s examine their most common use cases.

What is SCADA used for?

Industrial facilities such as oil and gas factories commonly use SCADA systems to remotely monitor and control processing equipment effectively. The systems uphold operational safety and efficiency by monitoring pipeline conditions and detecting and acting on anomalies.

Additionally, SCADA systems can help manage inventory and energy consumption, improve maintenance, and more. They notify personnel when inventory is running low or machines need repairs, help optimize energy usage, and reduce costs. SCADA systems are also essential for troubleshooting by providing an overview of the entire system’s current state. This allows operators to identify potential issues and inconsistencies at any given time.

It’s also worth noting that SCADA systems are applicable in several other areas due to their inherent remote monitoring capabilities. They are also suitable for power generation and distribution, water treatment, automation and control of buildings (commercial/residential), and even agriculture. Moreover, logistics, goods manufacturing, public infrastructure, and many other sectors can benefit from SCADA.

How does SCADA work?

So far, it’s clear that SCADA systems enable organizations to monitor large-scale operations remotely. They intervene when issues arise and optimize overall productivity and efficiency. To achieve this, SCADA relies on a standardized architecture consisting of several key components.

SCADA architecture

Depending on the use case, the system architecture may differ. However, every SCADA system requires the following components to function:

Sensors and actuators: Sensors gather input data, like temperature or pressure levels, and transmit it to the SCADA system. Actuators are machines that perform physical action based on commands from field controllers.
Supervisory computer: The supervisory computer, called the SCADA master computer, is an integral part of a SCADA system. The computer connects to, communicates with, and controls all remote devices and terminals. Its primary role is to collect sensory data, analyze it, make decisions based on that analysis, and send actionable commands to the field controllers. Organizations may also keep a secondary supervisory computer in standby mode in case of an incident with the first computer.
SCADA field controllers: These devices connect to the sensors and actuators. Field controllers are an interface between physical analog equipment and a SCADA system. There are two common types of SCADA field controllers:
- Remote Terminal Units (RTUs): Typically, you can place these specialized devices in remote hard-to-reach locations near the process they control. RTUs collect and process sensor data and send it to the supervisory computer. Apart from communicating with sensors, they can also control actuators to a limited degree (typically without complex programming). RTUs are highly durable and ruggedly-built to operate in harsh physical conditions without a stable power supply or a high-speed wired network connection. They frequently use wireless radio communication.
- Programmable Logic Controllers (PLCs): These are usually computers that control industrial processes in local, controlled environments such as a factory floor. Like RTUs, PLCs gather sensor data and communicate with a supervisory computer, but they can also control actuators using more complex programming logic. Due to greater computing power, PLCs can support higher I/O (input/output) counts. Operators commonly use them in settings with structured, high-speed wiring systems, making them ideal for controlling processes in real-time. They are also rugged enough to withstand industrial conditions, though to a lesser extent than remote terminal units.
Human Machine Interface (HMI): HMIs are usually software applications that allow human operators or engineers to interact with the SCADA system by transforming and displaying information in a graphical format. This enables users to understand the data, monitor the process, and intervene if there are issues. HMIs can often be a part of supervisory computers.
Communications network: This represents the network connecting devices in the SCADA system and the communication protocols used to transmit data. This encompasses the communications between supervisory computers, sensors, actuators, and field controllers. Many older SCADA systems, developed before the standardization movement, use a variety of different proprietary communication protocols. In contrast, modern SCADA systems rely on open standards such as TCP/IP, enabling them to benefit from component interoperability. SCADA systems can leverage popular lightweight TCP/IP-based communications protocols such as Message Queuing Telemetry Transport (MQTT).

SCADA in action

With each component and its role in a SCADA system now clear, let’s see how they work together cohesively.

In a SCADA system, the process begins when sensors collect input data, which field controllers then retrieve. SCADA field controllers consist of a programmer logic converter that can be set to a specific condition or requirement. Users can program these controllers to check if an immediate action is necessary upon receiving the sensor data.

For example, if a machine overheats, the field controller can send a signal directly to an actuator to shut it down until it cools. If it doesn’t require an immediate action, or a field device lacks the intelligence to perform complex logic, the collected data routes from the field controllers to the supervisory computer via a communications network.

It’s important to perform specific preprogrammed immediate actions before communicating with a supervisory computer. This is especially true in critical situations where systems cannot tolerate latency from network communication. This is important to be able to respond to such situations, even in case of network outages.

Additionally, the supervisory computer analyzes data from the field controllers and uses that information to make more informed, intelligent decisions. If it makes a decision, it sends a command from a supervisory computer back to the field controllers (RTUs or PLCs). This activates the actuators to perform the instructed action.

The supervisory computer typically provides the HMI, providing human operators an interface with observability of all field data. Operators can monitor the connected devices, visualize and interpret the data, and manually intervene when necessary.

The communications network makes all these interactions and communications between SCADA system components possible. It’s recommend to leverage MQTT in your SCADA systems. It provides low bandwidth consumption, efficient data transmission, and reliable communications. To deepen your knowledge about the MQTT protocol, check out this comprehensive guide, or explore the MQTT Academy for engaging lessons and quizzes that demonstrate real-world applications of MQTT.

Additionally, you can experience the advanced capabilities of the Pro Edition for Mosquitto™ MQTT broker by signing up for a Cedalo MQTT Platform trial.

In essence, the following outlines a simple SCADA system workflow:

Data acquisition (by sensors).
Data processing (transforming sensor data to a representation understood by RTUs/PLCs).
Data transmission (from RTU/PLC to another PLC intermediate or directly to a supervisory computer).
Data processing (by supervisory computer).
Data presentation (to system operators, performed by supervisory computer via HMI).
Control (decision made by a supervisory computer or a system operator, which then communicates back to field devices).

The workflow above omits some details, like performing local decisions on the RTU/PLC level. Still, it captures the general flow of a SCADA system.

What are the types of SCADA systems?

There are four types of SCADA systems: monolithic, distributed, networked, and, most recently, IoT- and cloud-based system architecture.

Monolithic SCADA system

A monolithic SCADA system is the first iteration of a SCADA architecture. In these systems, end devices and sensors connect directly to a central computer. The first iterations might have been an analog control panel with gauges, LED-indicators, and knobs. This was common in early power generation facilities, manufacturing plants, industrial factories, etc.

Monolithic architecture made it hard to scale, maintain, and reconfigure systems due to the sheer number of end devices that had to be directly wired to a central location. At the time, SCADA systems were also closed and proprietary. So, organizations would rely on a single vendor’s support, restricting the ability to move data freely beyond the centralized computer.

It’s also worth noting that a monolithic system doesn’t necessarily require the control system to be near the sensors. They can be kilometers apart as long as a direct wired connection exists between them.

Distributed SCADA system

Distributed SCADA systems addressed the limitations of the first generation by interconnecting on-site SCADA facilities into a Local Area Network (LAN). This enables different devices within a relatively small area–typically 100 to 500 meters–to communicate with each other. However, beyond this range, the signal would require a boost.

This setup allows for installing intermediate devices (RTUs and PLCs) to gather data from numerous sensors. Then, aggregates and processes the data before sending it back to the supervisory control. With fewer direct connections, only RTUs/PLCs would need to connect to the centralized locations as they can manage other sensors and devices within their LAN.

Although the systems became more scalable and manageable, SCADA manufacturers continued to develop LANs. This created a lack of interoperability between devices since system components did not rely on open protocols. Additionally, SCADA systems did not prioritize cybersecurity at the time.

Networked SCADA system

By embracing open protocols and well-established networking technologies, third-generation SCADA systems enable interconnecting multiple LANs. They would establish Wide Area Networks (WAN) via phones or data lines, ethernet, and fiber optics, allowing the interconnection of different manufacturing sites into a single system.

This design allows SCADA systems to spread over a wide geographic area in a more cost-effective and scalable way–something that wasn’t feasible with earlier reliance on more expensive proprietary networks and protocols. End devices also shifted to adopting open protocols, improving interoperability and simplifying maintenance.

IoT-and cloud-integrated SCADA system

The latest iteration of SCADA integrates IoT technology, cloud computing, and big data analytics, making IoT SCADA systems more flexible, efficient, and scalable than their predecessors.

This design relies on a cloud backbone to decentralize control and monitoring, allowing a network of smart devices like servers, personal computers, and even smartphones to perform operations.

PLC vs RTU: What are the differences?

In SCADA systems, a typical term for edge devices that interface with sensors is Remote Terminal Units (RTUs), although some sources also define them as Remote Telemetry Units. Historically, you could program RTUs as terminal devices using relay logic, which required physical deinstallation and rewiring to modify.

In contrast, advancements in CPU technologies and the proliferation of microprocessors led to the invention of Programmable Logic Controllers (PLCs). These devices are industrial computers that can be easily reprogrammed on a software level. With time, PLCs started replacing dedicated RTU devices, where an RTU can also serve as a PLC and vice versa, though they aren’t necessarily the same.

In modern RTUs, distinctions between a dedicated RTU and a PLC can still exist. For example, RTUs emphasize relaying data and commands between sensors and actuators via wireless communications, and ruggedness to withstand extreme conditions, and focus less on extensive programming and control. However, PLCs can also be rugged, support wireless communication technologies, and operate in remote locations.

Therefore, it’s best to focus on the device’s intended functionality. As a rule of thumb, an RTU discusses a terminal (edge) device interfacing with analog sensors. In contrast, a PLC refers to a programmable computer that can control equipment and process data.

Clearly, the distinction between the two is somewhat blurred. As previously mentioned, some sources refer to an RTU as a “Remote Telemetry Unit” instead of a “Remote Terminal Unit,” likely focusing on their ability to gather measurements from sensors and transmit them to subsequent nodes in the network (e.g. supervisory computers or PLCs).

Also note that PLCs can work as data aggregators or so-called SCADA sub-masters. Specifically, a PLC may act as a central control node for a certain industrial location or geographic area and communicate with a central SCADA-master.

Another distinction is that PLCs come in different sizes: larger and smaller models than terminal units. Terminal units are typically compact, as they are designed to be edge devices placed at the farthest points in a network’s topology.

What are IEDs?

Another term you can encounter when exploring SCADA systems is IED or Smart Electronic Device (not Improvised Explosive Devices). An IED can be considered a highly advanced sensor with RTU and PID (Proportional-Integral-Derivative) control capabilities.

The latter grants IED properties to an actuator, allowing it to perform a specific action that aligns a deviating measurement to its norm. These are typically expensive modern plug-and-play devices. They can be installed to provide more detailed process measurements and eliminate RTUs/PLCs in the middle to make a system more compact and easier to install.

However, instead of replacing RTUs or PLCs, IEDs often complement them by serving as advanced sensors with additional functionality. This would be viable when an RTU performs certain logic or communications that an IED doesn’t support, or when connecting an IED to a central system is not feasible without interfacing with intermediate RTUs or PLCs.

Benefits and drawbacks of SCADA systems

Now that you’re familiar with SCADA systems, how they work, and how they differ, let’s look at the key benefits and drawbacks to consider when using them.

Benefits

High connectivity and scalability

SCADA systems can connect thousands of sensors in a manageable way, making it possible to receive detailed information on many operational facets of the underlying process they monitor. This is achieved due to advances in distributed and networked SCADA system types and a shift to using open protocols for interoperability across many devices and systems.

Extensive remote data access

With SCADA systems, you can easily access, process, and analyze data from different parts of your systems in real time from anywhere (not necessarily from an on-site location).

Automated observability and enhanced operational control

SCADA systems facilitate observability, allowing you to remotely monitor processes, equipment, and systems. Should any issue arise, operators can quickly identify and resolve them from a centralized interface.

By consolidating all the system data into a centralized control point, operators can make informed, contextualized decisions by observing the bigger picture. Not only does this improve overall efficiency, it also provides valuable insights into the system’s operations at any given time.

Historical data storage and improved data analytics

SCADA systems often have extensive data storage capabilities and can analyze historical trends and identify patterns in large amounts of data. This enables organizations to generate more detailed data analyses and statistics.

Drawbacks

While SCADA systems offer various benefits, there are also some drawbacks to consider, as explained below:

Complexity

Operating SCADA systems at scale means managing hundreds or thousands of sensors in distributed fashion, configuring RTUs and/or PLCs (including those in remote locations), properly aggregating and storing the data etc.

It’s also important to remember that PLCs may require writing custom programs suitable to your use case and operational needs. This task is difficult as it requires careful planning, design, and proper operation and maintenance by experienced and knowledgeable staff.

Additionally, although modern SCADA systems rely on open protocols, integrating them with legacy systems or specific third-party devices can still present challenges.

High installation costs

SCADA systems do provide cost savings in the long run. However, the initial cost of setting up your SCADA system can be high especially when using it for large-scale operations. The hardware, software, personnel, and specialized training all contribute to this high cost. It can even be a barrier for small and medium-sized organizations with limited budgets.

Security

While the newer iterations of SCADA systems can incorporate security features like firewalls and encryption, the complexity of installation may lead to overlooking certain security aspects, leaving the system vulnerable to cyberattacks.

The scale of such systems, remote components, and certain analog aspects introduce a broader attack surface making implementing security more challenging. Additional thorough care, certification, and regular audits must be exercised to ensure that SCADA systems are appropriately secured. It’s especially important when considering critical industries such as power generation and water management.

Using SCADA and MQTT together

Incorporating the MQTT protocol into your setup can alleviate some of the SCADA system’s drawbacks. The MQTT messaging protocol is lightweight and facilitates reliable data transmissions, secure communications, and scalability.

An example of how to incorporate MQTT with SCADA is to set up an MQTT broker, like the Pro Edition for Mosquitto™, to serve as a unified communication network. The main advantage of an MQTT broker is reducing installation and operational complexity as it is based on open protocols, has an active community, and features broad interoperability with different IoT devices.

Moreover, MQTT brokers are highly scalable, supporting thousands or even millions of connected devices while requiring minimal resources. This makes them an efficient alternative network component for your SCADA system. Even if your SCADA system doesn’t natively support MQTT, it’s still possible to incorporate MQTT by using middleware. You can sign up for a Cedalo MQTT Platform trial to get access to the Pro Mosquitto MQTT broker and enhance your SCADA architecture.

By using MQTT in your SCADA system, you can ensure the following:

Consistent, reliable communications: MQTT’s Quality of Service enforces a chosen level of delivery guarantee. This ensures critical message delivery even under unreliable conditions. At the same time, non-critical messages can be configured to be less resilient or allow fewer duplicates in favour of greater performance. Additionally, in case of network interruptions or outages, MQTT brokers allow persistent undelivered messages to be stored on disk and resume processing once the connection is restored.
Low bandwidth consumption: MQTT uses a binary format to minimize message size, reducing the amount of data transmitted over the network. This allows messages to be sent from the publisher to a subscriber more quickly, making MQTT suitable for networks with limited resources while streamlining communications.
Enhanced security: Using MQTT over TLS/SSL secures data transmissions and encrypts them in transit, keeping information shared between devices and the MQTT broker confidential and tamper-proof.
Simplified integration with IoT devices: MQTT is an open, standardized communication protocol that makes it easy to integrate with various IoT devices–regardless of brand or model. It allows new devices to be added or removed from the system more easily, in a plug-and-play fashion.
Improved scalability: The ability to construct a hierarchy of MQTT brokers enables the integration with PLCs/sub-masters to facilitate connections between devices in a specific location. These brokers can aggregate data by using custom control programs and communicate the results to a supervisory computer. This ensures better scalability and performance of the entire system. Simpler integrations with IoT devices also contribute to this point by streamlining device configuration and communication.

Wrap up

SCADA systems allow organizations to control operations remotely, monitor and analyze data in real time, and interact with various devices within the network. With SCADA, organizations can improve efficiency in their processes. They can make critical decisions through data analysis, and preemptively identify potential system issues.

Today, there are four types of SCADA architectures: monolithic, distributed, networked, and most recently IoT- and cloud-based systems. Typically, a SCADA system consists of supervisory computer(s) running HMIs, SCADA field controllers (RTUs or PLCs), sensors, actuators, and a communications network.

SCADA provides several benefits such as, high connectivity and scalability, remote data access, observability, and data storage for subsequent analysis.

The most crucial attribute of SCADA systems is the level of control they provide. Even if a SCADA system is distributed geographically, human operators can monitor critical devices and processes from a centralized location. If any issues arise or control decisions need to be made, SCADA enables operators to act swiftly and take necessary actions.

However, SCADA systems also have several drawbacks, such as complexity, high installation costs, and security. To alleviate these potential issues, it is recommended to leverage the MQTT messaging protocol in your SCADA system.

By using MQTT and SCADA together, you can enjoy efficient data transmissions, reliable communications, low bandwidth consumption, enhanced security, and, most importantly, simplified installation and integration with various IoT devices.

Take advantage of the Cedalo MQTT Platform trial to test the Pro Edition for Mosquitto™ MQTT broker to ensure secure communications within your SCADA architecture. For a deeper understanding of MQTT and its real-world applications, explore the MQTT Academy for guided lessons and quizzes.