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PLCs Explained: Programmable Logic Controller Basics


An introduction to PLCsPLC and Technician

PLCs are like the brains of industrial automation equipment. They control inputs and outputs behind the scenes making sure all is running as it should. PLC is short for programmable logic controller and it’s basically a computer designed to work in an industrial environment. It controls and monitors a number of inputs and outputs. However PLCs differ from normal computers in that a PLC is designed to run a single program in a sequential fashion. We’ll cover the basics and then go into how to choose the type of PLC that’s right for you.

Why and how did PLCs come about?

To understand the purpose of a PLC, we’ll need go back in time to the 1960s. Picture transistor radios, Barbie dolls, G.I. Joe action figures, and Chevrolet Impalas. Machine processes during this era were controlled through the use of electromechanical relays. We’re not talking about a few relays either, these systems took up entire walls. However, the size of the relay-powered machines was not the only disadvantage.

Other downsides included:

– Hardwiring had to be done in a very specific order.

– Time-consuming troubleshooting.

– Constant troubleshooting due to worn relay contacts.

– Strict maintenance schedule.

– Large and cumbersome.

– Changes involved entire system rewiring.

It wasn’t long before engineers began brainstorming solutions to these pain points. With the introduction of computer control in the industrial sector, the first version of the PLC was developed. The invention of the PLC is attributed to Richard Morley and his company, Bedford and Associates.


Relay Room

Example of a relay room before PLCs were invented.

What are the advantages of PLCs?

Richard and his team focused on ensuring maintenance electricians and plant engineers could easily understand and use this new programmable controller. Naturally, the process took much trial and error to get to a controller that solved all the previous pain points while still being user-friendly. Eventually, they were able to achieve this with the MODICON 184.

Nowadays, PLCs offer several advantages over the hardwired relays including: reliability, flexibility, cost effectiveness, speed, better communication, and advanced data processing functions.  However, the most unique advantage to PLCs is the ability to reprogram them.  Therefore, they’re highly adaptable to changes in processes or monitoring application requirements.

In addition, programmable logic controllers feature several diagnostic tools that facilitate easy troubleshooting such as:

– Report generation

– Fault Logs

– Step-by-step program simulation

– Manually run inputs/outputs


What makes up a PLC?

There are a several key components that make up a PLC. These include the CPU, inputs, outputs, and power supply. In addition, you’ll need a communication port, a chassis or rack, and a programming device such as an external computer. Depending on the type of PLC, many of these components may be combined into a single device. For example, a compact module may have a CPU with a set number of I/O and a specific communication port. We’ll briefly discuss each component and the options available for each.

CPU: The CPU is where the data processing, memory storage, and diagnostics take place. It requires a communication port which could be either an Ethernet, serial, or USB port. You’ll also need to choose a communication protocol (EtherNet/IP, Modbus TCP, etc.). Let’s also not forget about memory size. You’ll need to choose a size that will allow you to run the tasks you assign to the PLC.

Inputs/Outputs: The inputs and outputs relay information to the CPU. Depending on the type of input and output devices you plan to use, you have the options of analog, discrete, or specialty I/O.

Power Supply: This component either comes integrated with the base module, or it’s sold as a separate entity. Two of the most common voltage ranges available for PLCs include 12-24VDC and 110/220 VAC. Make sure the voltage range you choose is sufficient to power the CPU and I/O modules.

What Types of PLCs are there?

Unitary PLCs, also known as compact or integrated PLCs, are the simplest type of PLC and are suitable for simple processes. A unitary PLC will have a set number of I/O points and an integrated CPU with connection ports. Usually you can directly connect these controllers to the device or application in question.

Unitary PLC

Modular PLCs, also known as rack-mounted, give you more flexibility. You’ll still need a base module which will include your CPU, inputs, and power supply. However, you can customize the PLC by adding additional I/O modules. You can basically go wild here adding as many modules as the PLC allows. You could even insert an analog to digital signal converter.


Types of PLCs

How do I choose which PLC type I need?

Now that you understand the basic components of a PLC and the types available, you may wonder how to narrow down your choices. The first thing to consider is whether you’ll be implementing a new system, or adding to an existing one. If it’s a new system your options remain wide open. However, if there’s a system already in place, you’ll need to comply with that system’s requirements. Below are 6 questions to ask yourself when purchasing a programmable logic controller.

  1. Are you implementing this PLC into a new or existing system?
  2. What type of environment will the PLC reside in?
  3. What type and how many I/O points does your system require?
  4. How complex are the processes you’re looking to control?
  5. How will your PLC communicate with other networks, systems, or field devices?
  6. What programming language will you be using to dictate the PLC’s processes?


PLC Applications in the Real World

If PLCs are a relatively new concept to you, then it might be hard to imagine the range of possible applications for a PLC. Programmable logic controllers help to control system processes from a compact and structured location. Thus, these devices are applicable in a variety of industries. Some of the most recognizable applications include use of PLCs for road traffic signals, automatic car washes, elevators, automatic doors, and amusement rides.


Hopefully you have a newfound appreciation for the PLC. It’s definitely better than trying to manage hundreds of electromechanical relays. Convinced yet about the value PLCs can bring to your processes? We’re proud to offer you micro PLCs by Lovato Electric in our web store. If you still have questions regarding PLCs, please don’t hesitate to contact us.


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