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What is PLC? Programmable Logic Controller

Nowadays, it is pretty impossible to imagine an automation system utilized to control and monitor some processes without PLC (Programmable Logic Controller) or some other microcontroller-based device.

HMI+PLC System

If we go deeper into automation technology, PLCs are the basis for all other complex control systems specialized in different fields. The following paragraphs are dedicated to explaining PLCs, their operation, and all the benefits that have made them widely applicable and utilized in almost all modern industrial automation and control systems. 

Table of Contents

PLC History

To get a better understanding of PLC, let’s first look shortly into automation technology history.  Classical old-fashioned automation systems were realized as automation & control cubicles with many different components. Their basis consisted of different relays, contacts, fuses, switches, etc.

Relay Based Control System

The main characteristic of these automation cubicles was the fact that every automation function needed separated hardware, wiring, and location in the cabinet.

In other words, to implement some of the simplest control algorithms, you needed a cabinet with a bunch of elements, completely wired and located near the sensors and actuators.

More complex automation algorithms increased the complexity of such automation equipment, making the automation procedure quite tricky and unintuitive.

The design of control algorithms almost completely turned into pure wiring skills, so the specialized design tool – ladder diagrams, were developed by the PLC manufacturers and widely used.  

Programmable Logic Controller Definition

Under such circumstances, the main goal was to develop a robust device that would integrate several control functions and enable programmatically changing the control algorithms without changing the hardware and wiring.

This device should also be designed to operate reliably under challenging operating conditions such as dust, high temperature, increased humidity, etc.

The development of electronics, microprocessors, and overall computer technology resulted in the PLCs as robust, reliable, and flexible solutions utilized to tackle different automation tasks. As a result, there are various definitions used to describe the PLC.

The most common definition is stated by NEMA (The National Electrical Manufacturers Association), which defines the PLC as a “digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions, such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog I/O modules various types of machines or processes.”

Besides this official definition, many simplified by defining the PLC as a solid-state device in charge of controlling/manipulating and monitoring different sensors and actuators at very high rates.

PLC Architecture

All benefits provided by using PLC in process automation are directly driven by PLC’s flexibility, modularity, and specific Central Processing Unit architecture (CPU) combined with a specific CPU’s operating principle.

The modularity of PLC is expressed through different modules in charge of various significant tasks: Input/Output (I/O) modules, communication modules, and the CPU mentioned above.

PLC Architecture

I/O Modules

I/O modules are in charge of collecting information from different sensors and transferring control signals to the actuators.

There are a lot of different I/O modules specialized in various fields. Mainly, digital (binary) and analog inputs/outputs are commonly used as I/O interfaces.

Besides these standard I/O modules, there are a variety of specially designed I/O modules such as temperature modules, pulse counters, fast counters, position inputs, etc.


The communication role is dedicated to the specially designed communication modules. Depending on the considered controlled process and control system architecture, the communication tasks could be realized by the CPU, or independent communication processors could be introduced.

PLC Scan Cycle

The main difference between PLC and some ordinary PC is in the CPU. CPU in PLC is designed to tackle specific tasks by cyclic repetition-defined command sequences in very short periods.

Compared to the other microcontrollers, the PLC is characterized by a unique operating system running on a CPU. This operating system, specially designed to run cyclic commands in very short periods, is called Scan Cycle.

PLC Scan Cycle

The scan cycle begins with an input scan within which the PLC reads the contents of the input lines (input module registers). Next, the read data is transferred to a specific area of memory called the input image.

In this manner, the “picture” from the different sensors representing the current process state is replicated into the PLC’s memory.

The program scan is activated as the second part of the scan cycle. During the program scan, the processor executes program commands defined by some sequence of appropriate arithmetic-logic functions.

The data (operands) used in the program scan are extracted from the input image (if the operands are input data) or from the area where the internal variables are stored.

The processing results are stored in a particular area of memory – the output image.

It should be emphasized that during the execution of the program commands, data is not retrieved directly from the input modules, nor are the results of command execution directly written to the output devices.

Instead of directly reading and writing to the I/O devices, the program exchanges the data with the memory areas – input and output images.

Upon completion of the program scan, the PLC operating system activates the output scan. Within the output scan, the data from the output image is transferred to the output lines (output module registers).

The fourth part of the scan cycle is dedicated to the communication tasks. During this part of the scan cycle, the realization of data exchange with devices connected to the PLC is implemented.

After the communication scan, the operating system brings the PLC to the maintenance phase. This phase encompasses updating the internal clocks and registers, performing memory management, and other tasks related to system maintenance.

Usually, the user is not informed about this maintaining sequence, but it regularly runs in the background as an essential part of the scan cycle.

As mentioned above, all parts of the scan cycle are running in very short periods, and the complete scan cycle repeats repeatedly.

Depending on the processor type, the input and output scan cycles are executed in milliseconds.

The duration of the program scan inevitably depends on the program size – the number and complexity of the commands implemented as arithmetic-logic functions.

PLC Benefits

The overall benefits of automation based on PLCs utilization are mainly manifested as the following advantages: compactness, robustness, the ability of self-diagnostic and monitoring functions, easy extensibility of control sequences – programmability.

The compactness of PLC significantly reduced the necessary space and wiring requirements. This advantage is precious in high-risk environments with limited space access requirements.

The robustness of control systems with PLCs is increased in different manners. First, the PLC replaces plenty of relays and automata, which are also more susceptible to failures.

Secondly, the self-monitoring ability of PLC significantly decreases the time necessary to diagnose and solve errors occurring in PLC or its modules.  

The programmability of the PLC represents the most significant advantage. Instead of expensive wiring changes and adding new relays (and other hardware), any change in the control sequence on PLC could quickly be introduced only by changing the program code.

This ability caused the broad penetration of PLCs in almost all modern automation systems. As the first PLCs were designed to replace old-fashioned relay-based systems,

A PLC programming tool is also developed to look like old relay diagrams to be intuitive to the users without programming or software engineering experience.

PLC Programming Languages

The most often programming tool used to program PLC is ladder diagrams. The ladder diagrams practically consist of elements – coils and commands representing inputs/outputs. The sequence of ladders makes the control sequence, i.e., the PLC program, which finally implements the desired control scheme.

Besides Ladder Diagram (LD), there are also other PLC programming Languages such as:

  • Sequential Function Chart – SFC
  • Structured Text – ST
  • Function Block diagram – FB

These PLC programming Languages are less intuitive than the ladder diagrams, but they have significant advantages in large-scale systems with numerable I/O modules and large control sequences.

PLC Today

During the past few decades, PLCs have been developed to improve operating performances and suit different control tasks.

The development of solid-state technology also affected the PLC’s performances and abilities.  Modern PLCs are capable of coping with extremely complex control tasks and quite a high number of input/output signals.

Despite the significant development, the main premises represented through the scan-cycle stay more or less the same, providing the robust cyclic execution of the defined commands.

Ultimately, the economic aspect of PLC’s story could not be avoided. With valuable savings in equipment and wiring, the PLC programming and commissioning make the PLC-based automation state of the art.

PLC programmers do not need specific software engineering knowledge or relay technology knowledge. They need specific knowledge highly correlated with the considered PLC device and/or series of devices produced by some manufacturer.

Rockwell Allen-Bradly PLCs
Rockwell PLC
Siemens PLCs
Siemens PLC

From this point of view, once trained, PLC programmers can perform various programming tasks implemented independently of the specific operating environments and controlled processes.

The market of PLCs and auxiliary automation equipment abound in a wide range of products offering different devices from the home automation range to the devices developed for high-risk operating environments.

With a wide range of products, the market also requires and offers different knowledge levels, so PLC programming became a highly-demanded and valuable profession.

Who is using PLCs?

PLCs are used in many different industries and settings, including:

  • Agriculture – PLCs can control all aspects of farm management, such as the amount of crop produced, efficiency, and usage of planting equipment such as irrigation.
  • Manufacturing – PLCs perform routine tasks such as setting up the machines to manufacture a particular product or adjusting for production line downtime due to maintenance.
  • Utilities – A common use case for utilities is time-of-use billing systems that utilize load profiles generated with PLC programs.
  • Commercial buildings – lighting and HVAC systems in commercial buildings are often controlled by PLCs. These devices can also monitor temperature, humidity, and other environmental factors to detect specific health concerns.
  • Industrial machines – PLCs are used to operate specific equipment such as packaging machines or robotics that require high precision.
  • Oil and gas – PLCs perform complex tasks for oil and gas industry activities such as deep-sea exploration, flow control, and drilling.
  • Transportation – autonomous vehicles such as aerial drones and autonomous ships utilize PLCs to coordinate navigation, communication, and obstacle avoidance functions.
  • Lighting – PLCs are utilized to control the lighting system in commercial buildings. This includes changing brightness for various tasks, such as maintaining illumination levels while saving energy.
  • Retail – PLCs reduce cost and errors in the retail process by allowing the store manager to set up various tasks such as inventory management and handling returns.
  • Hospitality – Front desk computer systems at hotels often have functions controlled by an embedded PLC system. The rise in popularity of self-check-in kiosks has also led to embedded solutions due to resource constraints with staffing front desk staff.
  • Virtual instrumentation – the IT infrastructure powering the internet is essentially virtualized through PLCs. PLCs allow companies to create and manage virtual environments and use them for various purposes, such as testing new IT products without affecting the live environment.


PLCs are a cost-effective option that can be used to replace older systems and reduce installation costs.

The self-monitoring ability of the device reduces downtime for maintenance and increases its robustness compared to other control options.

Modern PLCs have increased programmability, leading to their widespread adoption across many industries where automation is desired.

Your next step in the automation and control journey will be to jump into our four parts, PLC Training For Beginners.


PLCs have been used in various industries worldwide, including manufacturing, hospitality, medical devices, IT infrastructure systems, etc.

The primary function of a programmable logic controller is to replace the function of manual switches by automating or programming it with specific actions.

Using an industrial programmable logic controller reduced the number of wiring changes required in automation systems minimizing costs involving rewiring and adding new relays.

PLCs are typically more advanced than traditional electromechanical relays because they can be reprogrammed as many times as needed without changing their makeup like a relay would need to happen with each change.

Because it is more flexible and easy to change, PLCs are also considered a cost-effective solution as they help reduce installation costs by replacing older systems.

Their self-monitoring abilities also lead to increased device robustness. In addition, these advanced control systems can automate processes without human assistance, which helps increase productivity and accuracy.

No, PLCs are designed to be easy for everyone to use. Anyone with some on-the-job experience in electrical work can quickly learn how they work.

Work with a PLC programmer or consultant to get the system up and running. Once configured, it will require almost no maintenance, which is an excellent advantage for those who don’t have a lot of technical expertise.

You can, and it’s pretty simple. You can also use PLCs to control lights and other devices in the house like garage doors and heating systems.

Programmable logic controllers are designed for flexibility and customization. The cheapest PLC available will depend on which manufacturer or brand, but they usually range from $100 for the most affordable to thousands of dollars for the bigger brands.

It depends on the person, their level of commitment to the process, and how much time they have to dedicate to it.

Some people may need a few days of concentrated work, while others may need a couple of months or more.

James Clerk Maxwell filed the first patent for a feedback control system in 1868 (U.S. Patent No 325631). German engineer Fritz Heidolph developed one of the earliest digital programmable logic controllers in 1947 that has been considered an ancestor of the modern-day programmable logic controller (PLC) device.

Finally, The actual inventor of PLC is not just one particular person, but a team led by GM’s engineer Dick Morley in 1968. It was then called a “standard machine controller.”

PLCs control or refine the process of manufacturing/production, and SCADA mainly reports on equipment status.

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Check out our new course:

PLC Programming

with Studio 5000 Logix Designer

Check out our new course: