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Programmable Logic Controllers are everywhere. Petrochemical facilities, large manufacturing plants, breweries, and even doughnut shops use PLCs. Since there are endless applications of PLCs, there is a steady demand for PLC programmers. This series of articles aims to introduce you to PLCs and PLC programming basics. First, we will provide an overview of what PLCs are and where they came from. After this, we will move on to basic programming, ladder logic, and how to get started. Lastly, we will look at more advanced programming with real-world examples. We hope to give you everything you need to get started with PLCs. In addition to these articles, you can learn more about PLC programming by taking our course. There we cover the basics in more detail and then move on to everything you need to know to begin a PLC programming career.

What is a Programmable Logic Controller (PLC)?

A programmable logic controller (PLC) is a special type of computer. Even in rough industrial environments, PLCs are incredibly reliable. A PLC is a general-purpose controller that uses data taken from a collection of inputs, processes them through a series of pre-programmed logic, and sends a physical output based on the results of that logic. Consequently, these controllers can automate the function of a machine, a process, or an entire manufacturing line. They function as the “brain” of a control scheme. It can read the inputs wired to it and “decide” what needs to happen once it has been properly programmed. Based on this decision, it then manipulates the output signals. Connected to these output signals are equipment located in the field. This could be something like a start/stop switch on a compressor. 

Benefits of PLCs

As a result of being programmable, PLCs provide great flexibility. If you want something in the control system to change, you can alter the programming on the PLC and it requires no changing to the physical inputs or outputs. This helps keep things simple and easy to troubleshoot. Controllers can be used for simple operations such as starting/stopping a pump. They can even handle complex ones like surge control on a multi-stage compressor. Because of this, modern PLCs can provide virtually any control method imaginable.

Brief History of PLCs

Before the advent of the microprocessor, machines were controlled using physical relays, switches, pushbuttons, etc. An engineer would draw wiring diagrams showing how the components would be connected. Due to the nature of this control method, there were many problems. The relay circuit wiring would grow more and more complex. Subsequently, This lead to troubleshooting becoming an extremely difficult chore. If you have several motors that need controlled, you could control the on/off status using relays. More relays would then be used to decide whether the motor CAN be turned on/off based on process conditions. The number of relays continues to grow and manufacturers began to amass entire cabinets of relays.

Large cabinet full of an overwhelming number of relays. Demonstrates why PLC programming is needed.
Courtesy of Signalhead via Wikimedia Commons

Problems with Relays/Diagrams

Due to the complexity in these wiring diagrams, they would often not be updated. They could contain hundreds of different wires. The wires must be in a very specific order to have proper control. If one relay was out of place, it could cause the whole control scheme to fail. If anything in the logic changes, the relays would need to be rewired. This provides another opportunity for a mismatched wiring diagram. The difficulty of troubleshooting and the risk of even a single relay failure were too much. There was a need for a better method.

A Solution

The solution came in 1968. The microprocessor had just been invented and was being used in the first PLCs. Only the inputs and outputs would need to be hardwired in this new controller. The PLC’s processor would handle the rest of the logic. Now, whenever there was a change to the logic, it could be done in the programming. There was typically no need to change the physical wiring. Consequently, troubleshooting became much easier. As PLCs continued to evolve, they grew smaller and smaller and more capable in handling complex operations. This resulted in them going from simple operations like basic relays, timers, and counters, to more complex ones including one-shots, analog capabilities, and PID functionality.

Types of PLCs

There are many manufacturers of PLCs and each one has a range of models available with their unique traits. Most PLCs can be classified by how they handle their input and outputs. Common types of PLCs include fixed and modular, and semi-modular.

Fixed (Monolithic)

A Monolithic style General Electric Series One PLC. Being a fixed PLC, it contains no expandable modules.
Monolithic General Electric Series One PLC

Fixed PLCs are monolithic. They have a given set of inputs and outputs that cannot be changed. This results in them being cheaper than their modular counterparts. Monolithic PLCs do have their disadvantages though. If one of the I/O channels becomes damaged, the entire PLC must be replaced. Also, since the number of I/O is limited, so is the possibility of expansion. As a result, these PLCs are typically found on smaller-scale processes with few I/O.


A modular style ControlLogix 5000 PLC from Allen Bradley. These modules allow for expansion of the PLC.
Modular type PLC. ControlLogix 5000 system by Allen-Bradley

Fully modular PLCs are more expensive. They do however offer many benefits over the cheaper fixed versions. The power supply, processor, and I/O cards are separate modules. They all attach using a shared back-plane. Cards can be digital or analog I/O, communication, or something else. This potential for expansion gives the user more versatility in their design and eliminates the single-channel failure issue the fixed PLCs suffer from. If a channel fails, the card can be replaced. There is no need to replace the entire PLC. Many PLCs even offer the ability to hot-swap cards. The operator can remove the card and replace it without needing to power down the PLC. You should not assume, however, that all modular PLCs are hot-swappable. If you try this on a PLC that is not hot-swappable, you risk damaging the cards or PLC.


A semi-modular style PLC from manufacturer KOYO. It contains both fixed inputs and allows for additional module attachments on the left-side.
Semi-Modular Koyo CLICK PLC

There do exist hybrid types of controllers that contain features of both. The PLC shown above has a fixed number of I/O built into the processor module but allows for expansion by attaching additional modules by plugging them into the right-hand side of the PLC.  Others have fixed I/O, but also a few slots for expansion. The PLC shown below has fixed I/O, but it also has expansion slots. These slots leave room for another analog input card. This card allows the PLC to read temperature using thermocouples.

In our next article, we will delve deeper into the programming side of PLCs. We start by first taking a look at some simple logic. Next, we go into ladder diagrams and examine some of the basic functions. We hope you enjoyed this article and look forward to seeing you in part II.

Post Author: CSA Admin

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