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PLC Basics

Our modern society, relies heavily on automated machinery or equipment to perform most of the critical industrial processes, from generating electricity to refining oil. Advancements in automation technology has led to the development of more sophisticated and tech-savvy industrial control systems. This has allowed industries to scale new heights of innovation and production. The Programmable Logic Controller (PLC) forms the backbone of these advanced control systems. 

What is a Programmable Logic Controller? What are the components that make up a PLC system? How do these controllers enable industrial automation? What are the pros and cons of PLC systems? What are the major applications of PLCs?  If you’re asking these questions, join us as we take a comprehensive look at PLC basics in this article. 

What is a PLC?

A Programmable Logic Controller is a special-purpose computer designed to automate different industrial processes and electro-mechanical systems. It consists of a microprocessor that is programmed by the user through a programming device like a computer connected to the PLC via a LAN cable. Once the program is downloaded to the PLC, it is stored in the non-volatile Random-Access Memory (RAM) in the processor.

PLCs are designed and built as rugged control systems, that are capable of operating reliably under harsh environmental conditions- such as extreme temperatures, humidity, wet, dry, and /or dusty conditions. For this reason, these controllers are often used in industrial plants like manufacturing plant assembly lines to control motors, circuit breakers, pumps, fans, lights, and other machinery. For more details on the functionality and applications of PLCs, keep reading on.

History of PLCs

To have a better understanding of the usefulness of PLCs in automating industrial processes, let’s look at a brief history of these controllers. Automation of industrial processes began long before the invention of PLCs. From the early to mid-1900s, industrial automation was mainly carried out by complicated electromechanical relay circuits. However, these relay circuits required an enormous amount of wiring and space for even simpler automation tasks. Also, making even minute configuration changes necessitated rewiring of the entire relay circuitry.

In response to the shortcomings of relay-based control systems in industrial automation, Dick Morley invented the first Programmable Logic Control in 1968. But it wasn’t until 1973 when Modicon developed the first successful commercial PLCs, to replace the complicated relay circuitry in automobile industries- General Motors and Landis Auto Ltd. The newly developed PLC systems were able to drastically increase the functionality of industrial control systems while reducing the required control-cabinet space.

Since 1968, Programmable Logic Controllers have revolutionized the industrial sector. Today’s PLCs provide a wide range of control functions including counting, timing, comparing, calculating, and processing various discrete and analog signals.

Key Features of a PLC

You can think of a PLC as the Personal Computer (PC) or the laptop you have at home. As they both have a Central Processing Unit (CPU), Memory, Inputs and Outputs(I/O), Power Supply and Operating Software (although with the PLC it’s a different O/S). Let’s look at the hardware and advanced features of a PLC.

PLC Hardware Components 

Every PLC you will come across will always have the following hardware components:

PLC Hardware Components  
  • Central Processing Unit: The internal structure of the CPU depends on the type of microprocessor used, which can be either 32-bit or 16-bit. Generally, a CPU consists of the Arithmetic and Logic Unit (ALU). The ALU is responsible for data manipulation and carrying out arithmetic functions such as addition and subtraction, and performing logic operations of OR, NOT, AND, and EXCLUSIVE-OR. Also, the CPU has the Control Unit that is used to regulate the timing of the PLC operations.
  • Memory: The PLC memory is located within the processor and is used to store the logic involved in program execution. PLC systems use two types of memory namely, Read Only Memory (ROM) and Random-Access Memory (RAM). ROM location can only be read, it cannot be written. It is used to store fixed data and programs used by the CPU. For instance, the PLC operating programs are stored in ROM. On the hand, RAM stores the counters, timer values, and information from the connected input/ output devices and other internal devices. Both the input data and the user-defined programs are stored in the PLC memory to be accessed during the program scan.
  • Input/Output Section: Have you wondered how a PLC senses physical parameters like flow, pressure, temperature, etc.? Well, the input module interfaces the input devices with the PLC’s processor. These modules provide information to the CPU, by monitoring field input devices such as sensors, switches, and start/stop push buttons. Whereas, the output module interfaces the PLC processor with the output load. These modules give the PLC control over the field output devices like pumps, motors, electric heaters, relays, and solenoid valves. The I/O modules can either be analog or discrete and due to their functions, they are often said to connect the PLC to the real world (the rest of the machine).
  • Power Supply Module: The power supply is essentially responsible for powering up the PLC system. It receives line voltage/AC power, often 120 or 240 VAC (volts AC), and converts it to a lower DC voltage, mainly 24 VDC (volts DC). The converted DC voltage is then transmitted to the PLC circuitry to power the rest of the PLC components. 
  • Programming Device: This device is used to feed the user-defined program into the processor’s memory as discussed above. It is also used to monitor or make changes to the values stored in the PLC program. In addition to these components, a PLC system can also incorporate an operator-interface device like a Hand-Held Monitor (HHM); for simplifying the monitoring of the system or process being controlled by the PLC.

Advanced PLC Features 

  • Communications: Buses are used as communication lines within the PLC. They transmit information in binary form-as a group of bits; in which a bit is a binary digit of 0 or 1, indicating ON/OFF states. Also, in addition to connecting to field input/output devices, a PLC might be required to connect with other control systems. For instance, users might need to export application data collected and processed by the PLC to a Supervisory Control and Data Acquisition (SCADA); the system which monitors a variety of connected devices even in remote locations. For this reason, PLCs provide a variety of communication protocols and ports in order to communicate with other control systems like SCADA. 
  • Human-Machine Interface: In order for the users to interact with the PLC control system, the HMI module is required to provide Graphical User Interfaces (GUIs). The HMI operator interface can be a simple display with a keypad and text-readout or large LCD (Liquid Crystal Display) touchscreen panels with more features. These interfaces enable the operators to review and input operational data to the PLC in real-time. 

In today’s world of Industry 4.0 and the Industrial Internet of Things (IIoT), modern PLCs are also equipped with advanced features to allow them to communicate data via Web browser, or connect to cloud data via MQTT (MQ Telemetry Transport), and even to databases via SQL (Structured Query Language).

PLC Programming

Whenever you’re using a PLC, it is important you design and implements concepts depending on your specific application. To accomplish this, you will need to program your PLC. The PLC program is usually written on a programming device like a computer, then downloaded to the controller.  

A PLC program comprises a set of instructions and commands, either in graphical or textual form. These instructions represent the logic that governs the system or process controlled by the PLC. PLC programming languages are classified into two main categories which include: 

  • Graphical Language: Including (i) Ladder Diagrams (LD) (i.e., Ladder Logic); (ii) Sequential Function Chart (SFC); (iii) Function Block Diagram (FBD).  
  • Textual Language: Namely (i) Instruction List (IL); (ii) Structured Text (ST or STX) 

All of the aforementioned languages are standardized for PLC programming by the International Electrical Code’s IEC 61131-3 standard. You can use any of the five languages to program a PLC, but graphical-based languages (like Ladder Logic) are highly preferred to the textual languages (like Instructions List programming); due to their visual appeal and ease of implementation. 

How Does A PLC Operate?

PLCs are complex and powerful special-purpose computers, but their mode of operation can be easily understood in simple terms; especially now that you’re familiar with their hardware components and basics of PLC programming. PLC operation involves four basic steps, which include:

A) Input Monitoring: The input modules connected to the PLC monitor and collect the relevant information from the field input devices, they then send that data to the CPU. During program scan, the CPU constantly checks the state of the input variables and processes the received information as pre-programmed data to be used in the next steps. 

B) Logic Execution: Usually, design engineers program PLC processors to recognize certain values and conditions, and to make appropriate changes to the states of the outputs based on the programmed rules. In the second step of the program scan, the CPU executes the user-created program logic and stores the execution results to be used during the next step.

C) Output Control: During this step, the processor updates the appropriate outputs based on the state of the input variables and the execution results of the programmed logic. This enables the PLC to control various output devices such as motor starters, switches, relays. With this logic, a wide range of program designs and functions can be implemented, enabling the PLC to control a variety of electromechanical systems and processes.

D) Internal Checks: In this step, the processor continuously runs memory routines to ensure that the memory is not damaged. It also performs internal diagnostics, by constantly checking its software and hardware components for faults. This step also includes PLC communications with programming terminals to avoid programming errors. 

In summary, the PLC’s processor receives information from connected field input devices through the input modules, processes the received data, executes the instructions from the stored user program, and triggers the appropriate outputs as per the received input data and the pre-programmed parameters. The CPU performs these functions in a repeating loop known as PLC Scan Cycle. Note, a PLC begins operation once the user has downloaded the program to its CPU memory and the appropriate Input/Output modules are connected. The diagram below shows a typical PLC in operation.

Types of PLCs

The two main types of PLCs include:

A) Compact PLC: Also known as Integrated PLCs, these are the simplest types of PLCs in which all the modules are contained within a single case with an integrated CPU that has connection ports. This type of PLC consists of a fixed number of I/O points and standard external I/O cards. Such that, every input and output module is decided by the manufacturer. Hence, it does not provide the user with the flexibility of expanding the modules; which makes it suitable for just simple automation processes. 

B) Modular PLC: This type of PLC is rack-mounted. It provides more flexibility as it permits multiple expansion through additional “modules”, hence the name Modular PLC. The PLC inputs and outputs are located in the I/O module, while the logic and arithmetic operations are performed by a separate CPU. The I/O modules can be located close to the CPU, or they can be quite distant-even in different buildings. Modular PLCs are easier to use as each component is independent of the other. With this PLC you can even insert an Analog to Digital Signal Converter (ADC). 

In terms of output, PLCs are classified into three types namely:

A) Relay output PLC: The relay output PLCs can be used to control both DC (Direct Current) and AC (Alternating Current) output devices. They are normally used when low resistance is required and they can control up to 2A(Amperes). 

B) Transistor output PLC: Transistor output modules are voltage-dependent and are only capable of operating DC loads. PLCs with transistor outputs are usually used for switching operations such as controlling lights and in low-power DC circuitry like inside microprocessors. 

C) TRIAC Output PLC: The term TRIAC refers to a solid-state electronic device known as a Triode for Alternating Current (AC). It is usually a silicon-based switch that is activated by a small control voltage from a controller like a PLC. TRIAC output PLCs function like MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistors. They are highly suitable for controlling low-power AC loads such as motor starters, lighting, and contactors.  

According to the physical size, PLCs can be classified as large PLCs, Small and Medium-Sized PLCs. While their processors can be divided into Nano, Micro and Mini processors. Some of the most popular PLC manufacturers across the globe include: 

PLC Manufacturer PLC Brand Name(s) Market Share Ranking 
Siemens Simatic 
Rockwell Automation Allen Bradley 
Mitsubishi Electric Melsec 
Schneider Electric Modicon 
Omron Sysmac 
Emerson Electric (GE) RX3iVersaMax (GE Fanuc) 

Advantages of PLCs

For many decades PLCs have been a standard element for industrial control applications. You may wonder, what makes PLCs such a popular choice compared to other controllers? Here is why:

  • PLCs are capable of carrying out much more complex control applications compared to hard-wired relay control systems. 
  • PLCs are fairly intuitive to program as their IEC 61131-3 standardized programming languages are simple. In addition, PLC programming is a mature technology with years of testing and analysis, and comprehensive tutorials on how to program and integrate PLC systems are readily available. 
  • PLC control systems are extremely versatile and scalable, and most PLC models can be used to control a wide variety of industrial machinery, processes, and systems. 
  • As PLCs use electronic communication lines like bus technology, they eliminate the need for interconnected wires, which saves on installation costs. Also, PLCs have smaller physical size requirements compared to hard-wired control solutions 
  • PLCs have integrated override functions and centrally available diagnostics. 
  • PLC systems have relatively few components, which makes them easier to troubleshoot and service; ultimately reducing maintenance downtime. Also, PLC components require relatively low electrical power, which helps to conserve energy. 
  • PLCs are exceptionally reliable solid-state devices with no moving parts, which could make the PLC system prone to wear and tear or other mechanical problems. They are also rugged in nature, meaning they can operate reliably even in extreme industrial environments.  

Drawbacks of Using PLCs

Every technology has its own shortcomings, and there are some applications for which PLCs wouldn’t be considered the best choice. Some of the potential shortcomings of using PLCs include: 

  • Like many other types of electronic devices, PLCs experience common electronic malfunctions such as communication failures and corrupted memory. PLCs are also vulnerable to Radio-Frequency Interference (RFI) and Electromagnetic Interference (EMI). 
  • PLCs are not suited for manufacturing processes that involve multiple analog inputs, as they have a low capacity of handling processes that involve multiple analog inputs rather than digital inputs. Also, they do not have the capacity to handle extremely complex data. 
  • PLC programming interfaces are less interoperable than one could expect them to be, given that all PLC programming languages are IEC 61131-3 standardized. This is due to the fact that different PLC manufacturers often use proprietary PLC programming software. 

PLC Applications

PLCs provide robust and highly flexible control solutions, as they are adaptable to almost all control applications. For example, based on the connected input and output modules, PLCs can be used to monitor operations in real-time and record run-time data such as operating temperature or machine productivity, or generate alarms whenever a machine malfunctions, or to automatically start and stop operations, and more. Here are just a few of the most common real-life applications of PLCs: 

  • Process Automation Plants: PLCs are widely used to regulate various mechanical processes and automation applications in manufacturing plants. By maintaining direct connectivity with field devices such as sensors switches, transmitters, and actuators. PLCs are able to control various automated processes. 
  • Glass industry: Since the 1980s, PLCs have been applied in the elaborate glass production process. They are used to monitor and gather data from analog and discrete input devices, and to provide quality and position control.  
  • Paper industry: PLCs are used to control machines that are used to produce paper products at very high speeds. For example, a PLC can be used to monitor and control the production of newspapers or book pages in offset web printing. 
  • Virtual modeling: PLCs can be used in virtual modeling systems to simulate how a new machinery or equipment part will perform when installed. 
  • Wind turbine operation: PLCs are being used to control wind turbines using input data from wind speed and direction sensors. They are aimed at making wind turbines more efficient and reducing downtimes. 
  • Cement Manufacturing: A PLC is used to ensure that the right quality and quantities of raw materials are mixed in the kiln during cement manufacturing. For example, a PLC program can determine which raw material outputs (quality and mixture proportions) can result in the best possible cement quality. 


Programmable Logic Controllers have adapted well to modern automated and manufacturing systems. Today, PLCs are considered fundamental elements in many automation and process control systems. In fact, PLC technology is still the most widely used industrial control technology on a global scale. Its prominence is expected to continue growing with the emergence of Industry 4.0 and the Industrial Internet of Things (IIoT). This shows that PLCs are here to stay, and a basic understanding of their functionality is very essential. This article introduces you to the basics of PLC systems, it explains why these controllers are so popular in the industrial sector and it provides a quick primer on how the operator. 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 8th, 2021 and is filed under Automation, Education, PLC, Uncategorized. Both comments and pings are currently closed.

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