SCADA – Supervisory Control and Data Acquisition


SCADA systems are the backbones of the automated industries of the modern world. An acronym for Supervisory Control and Data Acquisition, SCADA systems virtually are the show makers of industrial automation. Whether it’s your local supermarket, waste water treatment plant, refineries – you will find these systems running behind the scenes.

What is SCADA?

SCADA systems are industrial automation tools. These are industrial computers that monitor and control processes and workflows. SCADAs also monitor transformers, substations and other assorted electrical assets pertaining to the electrical utilities.

General Layout of a SCADA System
General Layout of a SCADA System

These systems run on software applications that are specifically designed for process control. They specialize in collecting real-time data from remote locations for controlling the conditions and equipment. This arrangement is fruitful for monitoring locations where human intervention is difficult, cost inclusive and impossible at times such as oil and gas rigs, telecommunication monitoring systems, transport, and waste & water control.


Just like a typical system, SCADA systems comprise of hardware peripherals and software. The various hardware paraphernalia is responsible gathering and feeding data into the computer where the SCADA software application is installed. The application software analyzes and processes this data and represents it in form of time interval trends.

SCADA also acts as an event recorder. All events happening are recorded and logged on a file. This can later be printed or saved onto external storage devices.

One of the extraordinary features of SCADA is that it is capable of identifying patterns over time. If these patterns sway from the normal trend, then the system raises an alarm. This feature is pretty beneficial in case conditions become hazardous. The alarms can allow human intervention which can prevent accidents from occurring.

SCADA controls and monitors the desired location such as a plant or the operating system of a regional facility from another centralized location. While monitoring, a number of communication over devices and central SCADA systems occur. These scattered units are special kind of sensors and Programmable Logic Controllers or PLCs. Information is sent via PLCs to the host SCADA system on the central location. SCADA then either logs this information if it is new or processes it for analysis, prediction and forecasting.

For instance, suppose a SCADA system is installed in a water filtration plant. The remote PLCs measure fluid pressure in the pipes and report readings over to the central system. This system may be located somewhere in the control tower. In case, the patterns are not identified, an anomaly is reported. The SCADA system will now alert the main control station via alarm of the problem along with other details such as anomaly severity and its progression over time.

These systems may vary with relation to the tasks they perform such as having simple functions of reporting temperatures of a building to achieving complex tasks like monitoring traffic and traffic lights.

Some of the main functions of SCADA are as follows:

  • Showing the current process state, also known as visualization
  • Displaying alarms and events from alarm log and logbook
  • Displaying trends, also referred to as Historian and their analysis
  • Keeping records of handbooks, inventory, data sheets and expert systems for documentation purposes
  • Communicating and data synchronization with other centres


A typical SCADA system comprises of the following basic components:

a) Master Station Computer Controls

These systems act as repositories of real-time reported data and information collected from the Remote Terminal Units or RTUs. They comprise of standard computer hardware peripherals.

Very few SCADA system designers have made their own computer paraphernalia, however, commercial off-the-shelf products are still in demand. The efforts of IT giants like CDC and IBM were short lived.

The hardware in these systems feeds data into the backend SCADA software. This is responsible for repeatedly polling the RTUs for data consumption. Once information is fed, the software then analyzes, processes, stores and retrieves upon request. Some of the processes include creating table catalogues, unit conversions and so on.

b) Human Machine Interface (HMI)

This is an important component of a SCADA system. The values that have been stored in the host systems are converted into a comprehensible and understandable part with the help of HMIs. These are then handed over to the human operators. The information from HMIs is used to provide diagnostic, managerial or trending results as well as detailed schematics that represent the current machine states. To make representations more understandable, they are presented in pictorial form.

c) Remote Terminal Units (RTU)

RTUs are normally sensors or transducers which allow electrical circuits to communicate with control equipment and process instrumentation. These are used to measure changes in physical parameters like temperature, pressure etc. The changes in these properties are measured via changes in the electrical characteristics of transducer components that indicate a physical change. A single RTU unit can indicate different parameter types. Depending on the incoming measurement values, the Input and Output circuit can be digital or analog. While the digital signal has a limited number of states and is mainly used for the purpose of flagging, analog signal on the other hand, corresponds to measurements within the numeric range of continuous values which can later be converted using an ADC. To control process equipment, specific signals are generated. Unlike its past predecessors, the RTUs of today are microprocessor based small devices. All these conversions take place internally in them.

d) Programmable Logic Controllers (PLCs)

With the help of microprocessors, the RTUs have become significantly smaller and smarter with increased efficient functionality. That being said, PLCs are a special type of RTUs and are built around the philosophy of automation. The PLC based RTUs have programmability as their biggest asset. These can be debugged as well as fixed on the field. Also, new features such as multiple polling support, time lagging, exception handling and reporting can be added on the field itself, which is very convenient. This also enables the RTUs in execution of simple logical processes, thereby eliminating the need to involve the master station. Since a lot of vendors have used many different types of communication methods as well as coding over the RTUs, there has been standardization of languages and protocols. For instance, the standardized control programming language is the IEC 61131-3. It is based on the intuitive approach rather than the procedural approach of languages like FORTRAN and C.

e) Communication

The data conveyance from an RTU to the master control and the commands sent from the central system to the RTU has to be done over a communication system. Moreover, factors such as speed, accuracy, performance, and security have to be consistent over the whole network, since the SCADA system may or may not be localized to a single point only. Before the advent of computer networking, most of the communication systems were voice based. These systems had the same limitations of bandwidth just like the other communication devices of the era. However, with the intervention of the corporate world, the systems were included over core networks and were therefore seamlessly integrated with LANs and WANs for communicating with the normal computer systems. This eliminates the need for creating a separate parallel network for SCADA systems only.

f) Telemetry System

In order to connect RTUs and PLCs with control centers, enterprise and data warehouses, a telemetry system is used. Telemetry can be wired and wireless. Some examples of wired telemetry are WAN circuits and leased telephone lines while those of wireless telemetry include licensed and unlicensed radio, Satellite, microwave and cellular.

g) Data Acquisition Servers

The data acquisition servers are software that use industrial protocols to connect field devices such as PLCs and RTUs with software services. This is done via telemetry. This helps the client can access data and information from the field using standardized protocols.

h) Historian

Historians are software devices that accumulate boolean events and alarms along with data that is time stamped in a database. This database can be used to generate the ongoing time patterns and trends in form of graphs in HTML. Historian is a client that requests the information to be fed from a data acquisition server.

i) Supervisory Systems

These computer systems acquire data which is being processed and controls the SCADA system.

Communication systems

There are several communication systems that can connect the supervisory system to the RTUs. In a SCADA system, an RTU does not follow intuition. It only follows orders and reports, without knowing what it measures. Only the master system knows what data is to be used with what and belongs to whom. There are a couple of protocols that need to be followed for this.

All protocols have two divisions, Master Protocol and RTU Protocol. The Master Protocol contains control statements from master to the RTU while the RTU protocol consists of instructions from the RTU to the main system. This ongoing communication between RTU and master becomes a base model for RTU to IED Communications (Intelligent Electronic Device Communications). One of the most popular of this is the International Electrotechnical Commission (IEC) 60870-5 series.

How These Systems Work

In the simplest of terms, the SCADA systems work with deploying multiple software and hardware entities. With the combined efforts of these elements, an industrial organization can:

  1. Gather, monitor and process data
  2. Control and interact with devices and machines such as pumps, motors, valves, etc. which are interconnected via HMI
  3. Record events with timestamps into log files.

In the most basic of SCADA frameworks, manual inputs or sensors send information to the programmable logic controllers or PLCs and remote terminal units or RTUs. This information then heads over to systems which have SCADA Software installed in them. The software analyzes and then displays the data for helping operators for reducing wastage. It also increases efficiency in the manufacturing process.

The process of data acquisition begins at the PLC or the RTU level. It comprises of meter readings and reports on equipment status that is forwarded to the SCADA upon requirement. The gathered data is compiled and formatted so that the HMI can take supervisory decisions to whether the RTU/PLC levels should be overridden or should be adjusted. The data can also be fed to Historian which is usually built on database management systems or DBMS for further trending and other insights with regards to analytical auditing.

Effective and efficient SCADA systems can lead to significant money and time saving.


The human machine interface (HMI) is the I/O device with the help of which a human operator can control the process. HMI presents the processed data to the operator in an understandable format. HMI is usually linked to the database of the SCADA system and its software applications. HMI provides diagnostic data, trending patterns and managerial information that include logistic information, maintenance procedures, detailed schematics and troubleshooting guides.

HMI typically works with presenting the information to the operating personnel in a graphic fashion, often in the form of mimic diagrams. This means that an operator can see a schematic representation of the control plant.

For instance, a graphic representation of a pump that is connected to a pipe displays that the pump is functioning. It also shows the amount of fluid being pumped through the pipe in the current moment. Once the desired level is reached, an operator can switch off the pump. With HMI, the software will be able to show the fluid flow rates increasing or decreasing in real time. Mimic diagrams consist of schematic symbols and line graphics that can be used to represent elements in the process. These can also comprise of digital photographs of the overlain process equipments made with animated symbols.

The drawing programs which the HMI projects to the operator for the SCADA system typically include drawing programs which can be used to change the representation of these points in the interface. The representations can be as simplistic as a traffic light on screen which represents an actual traffic light on field or as complex as the display from a multi-projector which represents position of all elevators on a skyscraper or trains in a railway network.

Alarm Handling

Another feature of a SCADA implementation is alarm handling. Based upon whether certain alarm system conditions are verified, the system determines when the alarm event occur. Once an event has been detected, certain steps such as activating one or more alarm indicators and generation of text or email messages for the informing remote SCADA operators occur. In some cases, an operator acknowledges the alarm event that deactivates the indicator while in a number of other cases, the indicators remain in the active mode until the alarm conditions are handled.

The alarm conditions can be explicit or implicit. For example, the point of alarm is a digital status point. It had values either NORMAL or ALARM. These are calculated by formulae based on other analog and digital points. In the implicit alarm conditions, consider the following instance – the value in an analog point lying outside is high or low is monitored automatically by the system. It then limits values that are related with that point.

Some examples of indicators of alarm include screen pop-up boxes, siren, flashing screen areas and so on. In all of these cases, the purpose of the alarm is to attract the attention of the administrator so that appropriate steps can be taken.

While designing the SCADA systems, when a cascade of alarm events occurs in short intervals of time, due care must be given. Otherwise, the underlying cause that may not be the earliest detected event can get lost.

Different Architectures

The evolution of SCADA systems can be observed in four generations.

There are as follows:

1) Monolithic: First Generation

The early computing for the SCADA systems was accomplished by large minicomputers as common network services did not exist at the time. As a result, these systems were independent with no outbound connectivity to other systems. Moreover, all the communication protocols which were used were strictly proprietary at that time.

Redundancy in the monolithic systems was achieved with the help of backup mainframe system which was connected to all the RTU sites and was used in during failure of the primary mainframe.

2) Distributed: Second Generation

Information and command processing were distributed across a number of stations. There were connected via LAN. The information that was shared was in near real time. Every station was responsible for their particular tasks. This made the cost and size of every station lesser than the systems of the first generation. The network protocols which were used were not standardized even till this time. They were proprietary and the security aspect was generally overlooked.

3) Networked: Third Generation

This generation was similar to a distributed architecture. In case of the network design model, the systems were spread across more than one LAN networks. These networks were referred to as PCNs or Process Control Network and these were separated geographically. There were several distributed architectures which ran in parallel. They had a single supervisor and historian.

4) Internet of things: Fourth Generation

During this time, cloud computing was commercially available. SCADA systems depended more and more on internet technology. This reduced the infrastructure costs significantly and increased the convenience of integration and maintenance. The systems report events in real-time. They also use horizontal scales of the cloud environments which implement complex control algorithms. Also, the use of the open network protocols provides better comprehensible and manageable security boundaries.

Security Issues

The SCADA systems which were designed for tying decentralized facilities such as oil and gas pipelines, power, wastewater collection systems and power distribution were robust, open and easy to operate and repair. However, these were not secure. The transition from proprietary technology to open, standardized solutions along with increased connection numbers and internet connectivity has made these systems more vulnerable to common computer security threats.

Some of the prominent areas for concern are:

  1. Lack of concern for authentication and security in the design, operation and deployment of existing SCADA networks
  2. The belief that these systems benefit from the security via obscurity with specialized protocols and proprietary interfaces
  3. The belief that physical security makes the systems more secure
  4. The belief that if the systems are disconnected from the internet, they are more secure.

With drift to open standards, while SCADA systems can now integrate easily with diverse industrial systems, however, they have also become prone to a variety of threats.

Some of these include:

  • System downtime
  • Denial of Service
  • Defamation
  • Trojans
  • Keyloggers for password stealth

All these issues call for dedicated security mechanisms for SCADA.

Application Areas

SCADA is a highly versatile and diverse hardware & software system which has many applications. Most modern day industries benefit from the monitoring and control abilities offered by this technology.

Some of the major domains that benefit from SCADA are as follows:

  • Energy
  • Manufacturing
  • Power
  • Oil and Gas
  • Foods and beverages
  • Recycling
  • Water Treatment
  • Waste Management
  • Transportation


SCADA systems ensure that the productivity targets are accomplished on time. They also ensure the smooth running of all the systems.

Foods and Beverages

SCADA systems in food industry control critical elements like temperature, movement of liquid and solid ingredients etc.

Gas and Electric Utilities

These systems are a great fit in the utility environments as physical products are moved across systems spread over large geographic areas. SCADA is responsible for controlling movement of gas and power through the distribution chains. They also micromanage the supporting telecom infrastructure.

Wastewater Treatment

SCADA systems control the functioning of the flow rate sensors and contaminant sensors in wastewater treatment plants. These sensors are vital to conversion of wastewater to drinking water.

Information Technology and Telecom

Although not widely popular, these systems are used in the telecom industry for a variety of processes. They are used for remote monitoring and system control. Some of their applications include monitoring server temperatures and gears, alarm contacts and guarding against physical intrusions.