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Industrial setups that deal with water and waste control, oil and gas, energy, telecommunications, and transportation, use complex technologies to monitor the processes involved. If the correct monitoring technology is not installed, then these industries would not be able to efficiently provide the essential services that people depend on.

Technological advances in the industrial sector have led to the emergence of the two most critical industrial control systems, namely Programmable Logic Controllers (PLCs) and Supervisory Control and Data Acquisition (SCADA) systems. These technologies work hand-in-hand to monitor and control equipment/ machines in process automation across the aforementioned industries. This article compares PLC versus SCADA control systems, mentioning the differences between the two including their basics as well as describing their working relationship.

What is a PLC?

Working Principle of a PLC

A programmable Logic Controller (PLC) is a device or a piece of hardware that is programmed by the user and installed in a control panel. In other words, a PLC is a specially designed computer that is used to control equipment/ machines and electromechanical processes in industrial setups.
When the PLC is installed in a system, its processor monitors the connected sensors or input devices, collecting data and critical information about the inputs condition and process flow.

On receiving this information, the CPU executes the user program in its application memory and processes the received information as pre-programmed data, which it then uses to perform basic control interventions. It performs those interventions by triggering the appropriate outputs when the pre-programmed parameters or conditions are satisfied. This process of continuously reading the field input devices, executing the stored user program, and updating the outputs accordingly, forms the basis of the PLC working principle, as illustrated in the diagram to the right.

A PLC is a versatile piece of hardware, is very flexible, easy to program, and fully scalable to an operation’s requirements. This means that it can function in a wide range of applications where advanced control options are necessary. Initially, PLCs were developed as heavy-duty programmable controllers to replace hard-wired relays, sequencers, and timers in the automobile manufacturing industry.

Today’s PLCs are designed as rugged automation controllers used to perform much more complex tasks. They are highly suitable for harsh environments in a variety of industrial processes. For example, a PLC system can be used to control complex industrial processes in extreme operating conditions, such as monitoring running machinery and motors inside the factory floor.

PLC Components

A PLC control system consists of four primary components including:

A) Power Supply: The power supply provides the necessary power required to run the primary components of the PLC. The common power levels used by most PLC controllers are either 24V DC (Volts DC), 120 VAC (Volts AC), or 240 VAC. Some PLCs have a separate power supply module. In addition to providing power to the PLC system, the power supply also monitors and regulates the supplied voltages and notifies the CPU in case of any anomaly.  

Hence, a separate power supply is an essential component for PLC systems used in industrial facilities that are prone to line voltage and frequency fluctuations. A standard PLC power supply should therefore be able to handle 10% to 15% voltage variations in the main power line, so as to provide well-regulated electrical power and protection to the other PLC components.

B) The Processor: Often referred to as the CPU, the processor is a solid-state device that performs tasks that are needed to complete the functions of a PLC control system. Being a microprocessor-based circuitry, the CPU comprises an arithmetic logic unit, a process image memory, program memory, counters, and internal timers. The PLC has an operating system that provides software support for the creation of a user program. The user program is then stored in the application memory, and when the CPU is performing a program scan it executes the user program to determine the necessary control instructions for actuating certain outputs. 

Hence, the CPU is referred to as the brain of the PLC system, because it controls and processes all the operations of the PLC system. But to accomplish this, the CPU needs to communicate with programming devices, Input/Output devices, and other PLCs. Note, the processor of a PLC operates on DC power rated at ± 5 VDC, which is supplied by the PLC power supply.

C) Input/Output Modules: These modules connect the PLC’s processor to the field devices. There are Analog I/O modules and Digital I/O modules. The Input modules transmit signals from the process sensors into the controller, they can be pressure sensors, input switches, and operator inputs. On the other hand, the Output modules are the devices that the PLC uses to communicate to the real world (to the machines or processes being controlled). They may be motors, pumps, relays, lights, etc. 

D) Programming Device: This could be a laptop or a desktop computer or a hand-held device used to run the PLC programming software. It receives signals and uses a Binary language (1 or 0) to process those signals and give a Binary output that can be understood by the CPU of the PLC. 

What is SCADA? 

SCADA (Supervisory Control and Data Acquisition) is a monitoring software, often used in conjunction with PLCs and other devices like Remote Terminal Units (RTUs) to monitor and run plant processes. Being a focal framework, the SCADA software is installed on a computer inside the central data monitoring hub of the plant. One of its main functions is to act as an interface with Human-Machine Interface (HMI) or industrial machines.

Also, as a supervisory framework SCADA enables users to track information from the industrial machines. For example, data from the PLCs and RTUs are relayed to the SCADA supervisory system, then, when necessary, users or operators can enter control commands into the HMI to regulate the industrial process being controlled.

Components of a SCADA System

A typical SCADA system is made up of the following components: 

Basic Components of a SCADA System
  • Supervisory Computers: They are used to collect data on the process from all remote locations across the operation. The SCADA supervisory computer system is also responsible for communicating with SCADA PLCs which are used as field connection controllers. This system also includes the HMI software that runs on operator workstations. The operator can then use the HMI interface to issue control commands to the connected field devices through the supervisory computer system.
  • Remote Terminal Units (RTUs): They connect to input sensors and output actuators, in the process. These units are interfaced with the supervisory computer system using a communication network. Simply put, RTUs are intelligent Input/Outputs that provide various parameters to the SCADA Master.
  • Programmable Logic Controllers (PLCs): They are connected to the input and output devices within the process, in the same way as the RTUs. They are also networked to the supervisory computer system using different communication interfaces. Compared to Remote Terminal Units, SCADA PLCs provide more sophisticated in-built control capabilities. Also, these PLCs can be programmed using one or more of the IEC 61131- 3 standardized PLC programming languages. Most of the PLCs in a SCADA system are programmed using Ladder Diagram (LD). The ladder logic functions duplicate in format hard-wired relays and conventional electrical schematics, which are largely understandable by technicians and electricians without prior training on PLC programming.
  • Human-Machine Interface (HMI): This is a graphical user interface (GUI) that presents plant information to the operating personnel in form of graphical diagrams, event and alarm logging pages, that are schematic representations of the process being controlled. The HMI is networked to the SCADA supervisory computers from which it gathers real-time data to provide trend graphs, mimic diagrams, and alarm displays to the operator. 
  • Communication Interface: This interface connects the RTUs and PLCs to the supervisory computer system, using either manufacturer proprietary or industry-standard network protocols. The SCADA communication infrastructure may have components directly wired together, and in other cases, it may have the components communicating through a network of radios. LAN network lines are used to interface monolithic SCADA systems for communication purposes. SCADA systems in larger industries use SDH (Synchronous Digital Hierarchy)/ SONET (Synchronous Optical Network) communication infrastructure. The SDH/SONET network interfaces transfer digital data through fiber optic cables. In today’s SCADA systems, the use of Ethernet/IP networks is being preferred over Synchronous Optical Networking (SONET). Also, the recently developed Internet of Things (IoT) cloud computing technology, is being widely adopted in industrial processes to enable the components within the SCADA system to communicate.
  • SCADA Programming: The SCADA programming in the master station or HMI is used to create maps and diagrams of the monitored infrastructure. The created diagrams depict vital information during a process or event failure, they also provide activation keys to act. Most commercial SCADA systems use standardized interfaces in programming, with the commonly used programming languages being C or other derived languages. 

The diagram to the right illustrates the six main components of a SCADA system.

Differences Between a PLC and SCADA System

The primary difference between a Programmable Logic Controller and a Supervisory Control and Data Acquisition system is the technology used. A PLC is a piece of hardware that can be picked up and physically inspected, whereas the general SCADA framework is software installed on a computer system, whose functionality is comparable to that of an Operating System like Linux or Microsoft Windows.
Also, a SCADA system is designed to operate on a much broader scope. Often termed as a plant’s overall control system, as it can monitor and gather information from each and every element of the system using its hardware and software components. On the other hand, a PLC can only focus on monitoring just one output element within the system.

The table below provides a summary of the major differences between a SCADA and a PLC system:

FeaturesSCADA SystemPLC System
DefinitionGenerally, it is a software used to monitor and control processes, by acquiring data from input devices and controlling field output devices even in remote locations.It is a solid-state device or a piece of hardware, that controls the output of a process through a user-defined program, that is mainly written in Ladder Diagrams
Input/OutputInputs and outputs are represented as graphical diagrams or imagesInputs and Outputs are represented in Coil contacts, Normally Open (NO), or Normally Closed (NC).
Objects/ComponentsEach SCADA object is defined using a specific tag or nameEach PLC component is identified using a unique address

Table 2.1: Differences between SCADA and PLC

What is the Relationship Between PLCs and SCADA?

Because SCADA and PLC technologies are so distinct, one might think that the two aren’t connected. But the two work together to form an automation system that supports safe and effective operation within a plant. The automation system is used to prescribe maintenance tasks, which form the core of predictive maintenance schedules. A SCADA-PLC system works like this: 

  • Data from sensors and other input devices on a singular resource is transmitted to the Programmable Logic Controller (PLC).  
  • The PLC processes the received data and translates it into a format that can be utilized by the SCADA software. 
  • Clients or plant operators access the processed data in form of graphical diagrams or images through the Human-Machine Interface (HMI) on the SCADA framework. 
  • On the off chance that the data crosses certain set limits or thresholds, a maintenance work request is created by the SCADA system. 

This means that PLCs require a SCADA system to perform their operations, while the SCADA software requires the data collected by the PLCs to perform their work. Hence, the SCADA system can be looked upon as the broad software framework which supports the overall control system. Whereas the Programmable Logic Controllers (PLCs) operate within the control system that SCADA oversees. In an industrial setup for example, once the PLC identifies any issues within the process being controlled, it notifies the SCADA supervisory system, which through the connected HMI presents the problem to the human operator so that it can be fixed. 

For instance, if the control system is monitoring a running turbine in a hydroelectric power plant. Whenever the turbines show too much vibration, the connected sensors will transmit that data to the PLC system. On processing the received information, the PLC will send it to the SCADA software as pre-programmed vibration data. The SCADA framework will then inspect the readout vibration data and determine whether adjustments to the turbine operations are necessary. If any changes are required, SCADA will then trigger a work order on the user end through the HMI.  

Alternatively, depending on how the SCADA is programmed, it may send control instructions or commands back to the PLCs, which will then actuate the required change. PLCs act as controllers and relay points for specific assets (like the turbine), while the SCADA software controls the entire process. 

Applications of SCADA-PLC Systems

Since PLCs are designed to be extremely robust, they are commonly used to monitor an extensive number of sensors and actuators inside a factory floor. This is because they can withstand rigorous industrial environments characterized by extreme humidity, temperature, electrical noise, and vibration. On the other hand, SCADA technology is preferred for monitoring processes and equipment that are spread out across a wide geographical area. Hence, a PLC is able to perform well locally while SCADA performs very well remotely.

Regardless of the technology differences, both SCADA and PLC systems are used to monitor and control processes or equipment across a wide range of enterprises. Utilized together, SCADA-PLC systems structure a programmed control framework for endorsing maintenance tasks and actuating control actions. As previously mentioned, in industrial settings, for example, PLC and SCADA systems typically work together to achieve a common control objective.

You are most likely to find SCADA-PLC control systems in Electricity Generating plants, like in a hydroelectric power plant where they monitor and control running machinery ultimately controlling the entire process of electricity generation. Also, Electric Utilities utilize such systems for power transmission and distribution. In Water and Sewage treatment plants, SCADA-PLC systems are used to monitor and control water levels, water flow, and water pressure in pipes. Those are just a few examples of SCADA-PLC applications, there are many more.


With industrial automation changing how the world works, state-of-the-art SCADA-PLC systems are now being used to control industrial machines and processes. In an effort to expedite productivity, reduce human interactions and ensure consistent outputs from the systems being controlled. This article provides the basics of PLCs and SCADA systems, it outlines the differences between them and describes how the two systems work together to accomplish control operations.

In summary, a Supervisory Control and Data Acquisition (SCADA) system is generally software that provides a data and decision-driven control framework. While Programmable Logic Controllers (PLCs) control systems depend on hardware and prompt connections to adequately monitor and control outputs of field devices, whenever the connected input devices like sensors change state. A PLC operates within a system that is entirely controlled by SCADA, this is made possible through proper communication interfaces. For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on November 1st, 2021 and is filed under Education, PLC, Uncategorized. Both comments and pings are currently closed.

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