Programmable Logic Controllers And How It Works

220px-Siemens_Simatic_S7-416-3A PLC or Programmable Logic Controller is a device built that can compute small amounts of information to ultimately control an assortment of machines ideally working on an assembly line. To truly understand how PLC’s came about you have to delve a little into the history of programmable logic controllers. In present day industrial automation factories they are controlled using a programming language most commonly called “PLC Ladder Logic” amongst others. Looking through the time glass at the history of programmable logic controllers we can come to appreciate the hard work that thousands of people put into the development of PLC’s up until now.

Where It All Started

If you go back in time to the late 1960’s you can see where it all started. The earliest stage of this story is very enlightening where we walk into an age that makes us thankful for what we have today. Try to imagine staring at cabinets that measure up to fifty feet long and are filled with hundreds and even thousands of relays, cam timers, and drum sequencers and dedicated closed-loop controllers whose ultimate goal is to control a single machine.

How Does It Work?

What happens when something electromechanical goes faulty? You have to troubleshoot it, now can you imagine that way back then when something went wrong they had to figure it all out on pieces of paper, eventually shut down the machine, move or relocate hundreds of wires, add some relays, debug the situation and guess what? You have to do it all over again. The amount of labour and man hours spent on something like this must have taken many sleepless nights.

These were the kind of problems that the Hydramatic department of GM (General Motors) were sitting with so in 1968 they developed a design criteria for a “standard machine controller” that not only had to replace the old relay system but had to:

  • Function in an Industrial environment with all its noise, dirt, dust, moisture, electromagnetism and vibration.
  • Simplification of programming and maintenance was key while adhering to the principles of the current state of programming technology.
  • Be durable and flexible like a computer but priced competitively with a like kind relay logic system.
  • Easy exchange of its components demanded that it be modular in form.

So GM issued an invitation out to competing Companies to come forth with an electronic solution to replace old relay systems and four Giants came forth to compete:

  • Digital Equipment Corp (DEC)
  • Century Detroit
  • Bedford Associates
  • Information Instruments, Inc

Bedford Associates won the contract and quickly renamed the company to Modicon – named after “Modular Digital Control”. Heading this was a certain Dick Morley who some claim was the father of “PLC’s” but in his Bio he reveals he was not alone.

After many years of development we presently employ the use of Scada and PLC technology to control machinery that manufactures almost everything we use today. The history of programmable logic controllers has transformed the sweat and tears of many men into the thriving industry it is today.

PLC Programming Basics

PLC stands for “Programmable Logic Controller” and simply put it is a small type of computer that is designed to control machinery in industrial factories on an assembly line as well as things like fun rides at amusement parks, so in essence anybody should clearly be able to appreciate the art of PLC programming because if it weren’t for these devices and the people who can program them we probably wouldn’t have all the modern day luxuries that we take for granted. Due to the fact that these devices can withstand harsh environments and weather they can last for many years. PLC programming is mostly a visual process when compared to conventional programming which basically involves sitting strictly in front of a computer.

It is because of this visual approach that makes the programming procedure quite straightforward and would definitely suit people with a preference to hands on work.

What Is The Ultimate Purpose And Function Of A PLC?programming_PLC

Its core function is to control machinery by simplifying the command process. They can also be considered of as a hard real time system because it is wholly dependant on programmable time restraints. A program is written for it using a simple software program that runs on a normal pc which is then transferred to the PLC via the appropriate cables and this program is then responsible for turning on and off outputs based on command lines given to it through the inputs of the internal program.

Once a PLC is programmed it can run and control the designated machine for many years without any human intervention. This is why PLC’s are built to be durable and tough so that they can withstand extremely harsh conditions and function on automatic thereby increasing the production volume. These PLC programs are then also stored in what people would call non-volatile battery backed-up memory.

Ladder Logic

The programming language used in PLC programming is closely termed as “Ladder Logic” because its ladder diagram design resembles that of a ladder. To understand this one must create a ladder diagram which function is the pairing of the vertical lines (rails) to the horizontal lines (rails), and whereby every rail has a different voltage and every rung is a circuit whereby the voltage runs through.

PLC Basics Of Operation

A PLC is designed to read specific analog and digital inputs and from a variety of little sensors and after processing it according to its logic programming can execute the instructions via the output values straight to the output machines or devices like solenoid coils, hydraulics, indicators and counters to name a few.
Most PLC’s operate on a scan cycle basis and the details of these vary from brand to brand.


Companies that employ the use and technology of PLC’s include Omron, Allen Bradley, Siemens, Messung and Mitsubishi.

PLC programming is not only used for machines in industrial settings but can also been seen being used in public friendly environments like internet shops and cuing systems in malls.

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