Programmable logic controllers were created in the 1960s. They have slowly taken over various industries due to their incredible utility and flexibility. In this article, we will provide a definition of the programmable logic controllers and present the basic elements it contains. Finally, we are going to offer a list of the various uses and applications.

What Are Programmable Logic Controllers?

PLCs (or programmable logic controllers) refer to industrial solid-state computers that have the role of monitoring inputs and outputs. They make a decision based on logic for various automated machines or processes. They have been part of factory automation and the process of industrial control for a long time now. A programmable logical controller can fulfill many functions, which is why they are so popular in the industrial world.

Brief History

Back in 1968, the automatic transmission division belonging to General Motors, namely the GM Hydra-Matic, asked for an idea. They wanted to replace the hard-wired relay systems starting from a white paper that had been designed by the engineer Edward R. Clark. The Bedford Associates company from Bedford, Massachusetts, came with the winning suggestion.

Thus, people managed to create the first modular programmable logic controller. The Bedford Associates later created a new company dedicated to this product, which was called Modicon (Modular Digital Controller). Richard Morley is considered the father of the PLC, having worked on the project right from the start. Ever since then, the automotive industry has remained one of the largest users of this product.

Programmable Logic Controller Basics

Of course, there are lots of programmable logic controller books out there, which can help you have an idea about their basics. But since they can be quite long and complicated, today we are presenting you a simplified rundown of the PLC basics.

How Does a Programmable Logic Controller Work?

Internal Structure

The brain of a PLC is a CPU (central processing unit). It is, in fact, a microprocessor that can be 16 or 32 bits. It’s made up of a memory chip paired with integrated circuits. They are used for control logic, for communicating, as well as for monitoring. The CPU orders the PLC to communicate with other devices, to issue internal diagnostics, to execute the instructions or to finish logic and arithmetic operations. It also checks constantly for programming errors and it makes sure that there is no damage to the memory.

plc internal structure

The memory offers a permanent type of storage for the data in use by the CPU mentioned above. There are two types of memory:

  • ROM (Read-only Memory) – permanently stores data for the OS (operating system)
  • RAM (Random Access Memory) – stores information about the status of the input and output devices, values of the internal devices or for timers and counters.

To upload the data to the CPU, the PLC requires a programming device such as a console or a computer.

Basically, PLCs have the role of reading signals sent by various sensors or other kinds of input devices. The input devices can refer to sensors, keyboards or switches. Of course, the inputs can be either analog or digital. The input modules are in fact robots or visual systems sending signals to the PLC, while the output ones refer to the motor or other solenoid valves. You can notice their arrangement in the block diagram of the programmable logic controllers.

Processes and Current

There are two important processes we need to clarify when discussing the way in which PLCs work, namely sinking and sourcing.

  • Sinking – the common ground line usually marked with a dash in diagrams.
  • Sourcing – the common VCC (voltage connection point) line, usually marked with a +.

The inputs received by the PLC for sinking and sourcing lead electricity only in one direction. In general, the input enjoys its own return line. Several inputs come together for one return line and not separate ones. The common lines usually bear the label COMM. The sensor outputs have the role of making how big the signal given is.

In what concerns the current, you should know that the AC (alternative current) inputs are less frequent than the DC ones. This happens because most of the sensors used here have transistor outputs. As such, if a system relies on a sensor input, most likely it’s going to be a DC one. In general, AC inputs require more time on behalf of the PLC to see, in comparison to the DC inputs. Lastly, you need to know that a regular AC input refers to a mechanical switch that is used for the slow mechanical drives.

Finally, one of the most common output connections for a PLC is represented by the relays. A relay has the purpose of switching the AC or the DC modules in case they are non-polarized. It moves rather slowly, and when it switches or settles it reaches a speed of 5 to 10 milliseconds. However, the bright side is that it manages to switch a large current.

common inpucts in plc

How to Use a Programmable Logic Controller

The first PLCs (up until the 1990s) were usually programmed with special-purpose terminals or proprietary panels. The programs were stored on cassette tape cartridges and people used the ASCII character representation. If you want to use a PLC, then you should know that there are 5 programming languages compatible with it. Many programmers choose to use an open source programmable logic controller, which is definitely better. Here you have the 5 programming languages that are used for the PLCs:

Ladder Logic

One of the most commonly used PLC languages, ladder logic uses lots of symbols. The symbols in it represent the counters, shift registers, timers, closing relays, as well as mathematical operations. The rules used in ladder logic are named rungs. For each rung, there is a single output. However, a single input can be found in various rungs.

Function Block Diagram (FBD)

The second programming language for the programmable logic controllers is the function block diagram one. It shows the functions between the input and output. The function contains blocks and connects the variables. This language is particularly useful for showing algorithms and logic, especially for the control systems that connect with each other.

Structured Text (ST)

This is a high-level language based on sentence commands. With it, you can use statements such as If/then/else or repeat/until if you want to create programs.

Instruction List (IL)

IL is a low-level language that relies on functions and variables. You can control the program with jump instructions and subroutines. The latter can have some optional parameters.

Sequential Function Chart (SFC)

The last language is, in fact, a method of programming complex control systems. It relies on basic building blocks which are able to run their sub-routines. It is great because it divides other large and more complicated tasks into smaller ones, which are thus more manageable.

Programmable Logic Controller Uses and Applications

Considering the importance of programmable logic controllers, you should know that there are lots of uses and applications for it. We are including here a rather brief list of the industries and places where PLCs have been used ever since their invention. However, in the future, the list may expand, regardless of the limitations of the programmable logic controllers.

  • Travel industry – For travel industries, there have been many uses of the PLCs. For example, they are useful in lift or escalator operations, as well as for monitoring safety control systems.
  • Printing industry – PLCs are used in the printing industry for multi-stage screen washing systems and registering control systems for the offset web press.
  • Manufacturing industry – You will find them in extruder factories and lead acid battery plants, for instance. There they complete the manufacturing system or the silo feeding control one.
  • Aerospace – This industry relies on a water tank quenching system where the PLCs are incredibly useful.
  • Hospitals – Here you will find a coal-fired boiler system that relies on a fan change-over.
  • Food industry – The filling machine control system used for many cakes and pastry products is one use of the PLCs. The water pump that brings water to the plant is also based on such a system.
  • Film industry – As surprising as it may sound, the PLCs are useful for the film industry as well. People working in this field use a controllable camera positioning system relying on a servo axis.
  • Agriculture – For glasshouses, the ventilation, watering, and heating systems all rely on the PLCs.
  • Leisure – Roller coasters make great use of these systems.
  • Textile industry – Textile shrinkage system and the washing machine control system.
  • Plastics industry – Injection molding system or a silo feeding control system in an extruder factory.


Many people are still not that quick into using this new technology. However, with some programmable logic controller training, everybody can learn how to use such a device. As you have seen, its uses and applications are overwhelming, which shows that it’s truly a reliable feature in today’s world.

In case your employees don’t understand the principles behind these systems, you can always use various diagrams or a programmable logic controller PPT presentation since visual hints are more efficient. All in all, next time you can be aware of what systems lie behind the roller coaster you see or the croissant you’re eating.

Image sources: 1, 2, 3.