Explaining HMI, SCADA, and PLCs, What They Do, and How They Work Together

When it comes to the process automation and advanced system design, PLCs, SCADA, and HMIs play crucial roles. A PLC is a hardware-based device, SCADA is a system that works in conjunction with the PLC. But, an HMI is also a system that works in conjunction with a PLC. Since the SCADA system and an HMI can complete more or less the same functions, it’s crucial that you know the difference between these two.

In this article, we’ll talk about each of them and explain their functions. Furthermore, we’ll discuss how they work together in a seamless system that controls different functions. We suggest that you grab a cup of coffee– it’s going to be a long read!

SCADA Systems

SCADA didn’t get its name by luck, it is actually an abbreviation that stands for Supervisory Control and Data Acquisition. As we said earlier, it’s a system that has both software and hardware elements and is usually used for a couple of things:

  • Control of various processes from a remote location or locally
  • Interaction with sensors, pumps, motors, and valves through an HMI (Human-Machine Interface) software
  • Monitoring, acquiring, and processing real-time data
  • Recording the events into a file.
SCADA System
SCADA System – researchgate.net

In the modern time, there isn’t a serious organization that doesn’t take advantage of SCADA systems. These systems help save some time, they increase efficiency, they process data faster and with accuracy, and they allow for smooth communication between various parts of the system. As such, the SCADA architecture consists of two basic parts – an RTU (Remote Terminal Unit) and a PLC (Programmable Logic Controller). A PLC is essentially a microcomputer that directly communicates with a plethora of objects such as end devices, sensors, or numerous factory machines.

A PLC, however, is directly hooked to the factory machine, while the RTU is used remotely, as its name implies. The sheer power of SCADA software allows us to process, distribute, and display the required data, thus making it easier for employees to analyze it and come up with solutions if needed. It works exactly as it sounds. If there’s an error somewhere in the process, it notifies the operator who can then pause the production process and check the data from the SCADA system. This data can be checked via an HMI, which we’ll talk about later.

Where is SCADA Used?

As we mentioned earlier, SCADA systems are mostly used in factories and industrial organizations in both private and public sectors. The reason they’re being used is to maintain efficiency and accelerate the data distribution. This further leads to the faster decision-making process and smarter use of time. Thanks to the versatile capabilities of SCADA systems, they can be used in both simple and complex installations. SCADA systems are used in energy, manufacturing, power, recycling, transportation, oil, and gas, as well as many other industries.

Not every SCADA system is greatly optimized. However, with the expertize of experienced engineers, SCADA systems can help save both money and time.


When it comes to the hardware-based part of the SCADA or HMI system, we can’t dismiss a PLC, as a crucial component in the system. PLC stands for Programmable Logic Controller and it is an industrial computer that is used for industrial automation. PLCs can differentiate in sizes and capabilities, depending on the company’s need. This powerful industrial computer works by continuously monitoring the state of input devices and makes decisions about controlling the state of output devices.

PLC Arduino Ethernet
PLC Arduino Ethernet – industrialshields.com

The great thing about PLCs is that they can control huge chunks of a production line, as well as the whole production line itself. Any kind of production line can be vigorously enhanced by using this technology, but the biggest advantage of PLCs lies in repetitiveness. This computer is able to replicate the operation or process over and over again, all while collecting vital information.

Because of its minuscule size, PLCs are modular and highly portable. This means that the PLC can be brought to the production line, hooked up to a computer, programmed, and put into work the exact same moment.

How do PLCs Work?

So, how do they work? Well, it is actually a very simple matter. So simple that you’ll be surprised how it manages to keep everything under control. The first step in the process is called an Input Scan. As its name implies, the PLC detects the state of all input devices hooked up to the PLC. After that, the second step is the Program Scan, which scans the program that the user created and then executes it. The next step is the Output Scan. As its name implies, it scans the output devices connected to the PLC and either energize or de-energize them.

Finally, the last step is Housekeeping. This step is more like a safety step in which the PLC communicates with internal diagnostics, programming terminals, etc. If the last step is done correctly and everything is under control, the PLC starts from the beginning until the loop is finished.

How are PLCs Programmed?

Even when they first came out, PLCs took advantage of Logic. Today, almost all PLC programming software offers an option to program in Ladder Logic. Ladder Logic is a traditional programming language that is used to mimic circuit diagrams that go from left to right. On the left side, the program consists of input contacts which can be closed or opened. An opened contact will conduct the current when pressed, while the closed one will stop the flow of the current when the signal initiates it.

On the right side, we have contacts of the output devices, which can be actuators, solenoids or valves. Usually, it’s a solenoid. When a signal reaches the input contact, it’s initiated and the condition (output) is fulfilled. The newest innovation is called “C” programming, but many programmers still use Ladder diagrams just because it’s easier to use and it’s been used for quite some time.


The reason we left HMIs for the end is that they are used to communicate with Programmable Logic Controllers and as such, the whole system is completed. Whereas SCADA represents a remote system used to communicate and collect data, HMI is a local machine capable of doing the same thing. But, the only difference is, as we said, that HMIs are local machines. The Human-Machine Interface is the user interface that connects the operator to a system, device or machine.

Human-Machine Interface
Human-Machine Interface – inductiveautomation.com

An HMI isn’t a particular piece of hardware but rather a screen that allows a user to interact with a device. HMIs can also be called Operator Terminals, Local Operator Interface, Graphical User Interface, etc. If some of these names (OT, LOI, and GUI) sound familiar to you, it’s because you’ve already used some of them. Simply put, HMIs are used for visualization of particular data, for easier understanding and control.

The prime modern example of an HMI is a tablet. A tablet is a GUI that allows you to control various processes. If you’re connected to your TV via Bluetooth, you can use your HMI (tablet or a smartphone) to control it. HMIs come in a variety of shapes and sizes – computer monitors and machines with built-in screens.

How Is It Different from SCADA?

As we mentioned earlier, SCADA and HMIs are almost the same, yet still different. Both SCADA and HMIs play a huge part in an industrial system that encapsulates them, alongside PLCs. However, they function in a different way. An HMI uses some amount of data and visually represent it, allowing for greater understanding and more efficient supervising process. On the other hand, SCADA systems are focused on control-system operations and they have a huge capacity for data collection. The main advantage of SCADA systems over HMI is that they collect and record information.


As said, SCADA and HMI systems are here to collect and display certain data, respectively. A PLC, as an industrial computer, serves as a bridge between the process and its control. As such, a PLC is often used in conjunction with SCADA systems and HMIs. An HMI is something that’s ever-present in every type of industry and even in our homes in a form of a monitor or a tablet.

As the technology progressed, new types of HMI and SCADA systems are made. The prime example of modern HMI is touchscreen, which offers a lot of flexibility for the operator. With the growing trends in industrial hardware, both SCADA and PLCs got some boost. PLCs are now able to perform the most complex logical tasks, while SCADA systems are much more versatile and are able to collect a huge chunk of data.

All in all, with the progress of technology, it will be interesting to see how these systems can improve even more.

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