computer science

Programmable logic controller

Programmable logic controller is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Programmable logic controller rather than just read about it. In short: A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. PLCs can range from small modular devices with tens of inputs and outputs (I/O), in a housing inte…

Key takeaways

Programmable logic controller belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Programmable logic controller to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Programmable logic controller from memory before moving on to harder problems.

Reference excerpt

A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. PLCs can range from small modular devices with tens of inputs and outputs (I/O), in a housing integral with the processor, to large rack-mounted modular devices with thousands of I/O, and which are often networked to other PLC and SCADA systems. They can be designed for many arrangements of digital and analog I/O, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. PLCs were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers which could replace hard-wired relay logic systems. Dick Morley is credited with having invented the first PLC, the Modicon 084, for General Motors in 1968. A PLC is an example of a hard real-time system: output results must be produced in response to input conditions within a limited time, so that unintended operations do not result. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.

Invention and early development The PLC originated in the late 1960s in the automotive industry in the US and was designed to replace relay logic systems. Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. The hard-wired nature of these components made it difficult for design engineers to alter the automation process. Changes would require rewiring and careful updating of the documentation. Troubleshooting was a tedious process. When general-purpose computers became available, they were soon applied to control logic in industrial processes. These early computers were unreliable and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality. The PLC provided several advantages over earlier automation systems. It was designed to tolerate the industrial environment better than systems intended for office use, and was more reliable, compact, and required less maintenance than relay systems. It was easily expandable with additional I/O modules. While relay systems required tedious and sometimes complicated hardware changes in case of reconfiguration, a PLC can be reconfigured by loading new or modified code. This allowed for easier iteration over manufacturing process design. With a simple programming language focused on logic and switching operations, it was more user-friendly than computers using general-purpose programming languages. Early PLCs were programmed in ladder logic, which strongly resembled a schematic diagram of relay logic. It also permitted its operation to be monitored.

Virtual PLCs In recent years, the introduction of virtual PLCs has expanded the scope of programmable logic controllers. Virtual PLCs are software-based controllers that simulate the functions of traditional PLCs but are executed on general-purpose hardware, offering a more cost-effective and flexible alternative. These controllers enable automation systems to be managed without the need for dedicated hardware, making them ideal for applications requiring simulation, remote control, or cloud-based systems.

Modicon

In 1968, GM Hydramatic, the automatic transmission division of General Motors, issued a request for proposals for an electronic replacement for hard-wired relay systems based on a white paper written by engineer Edward R. Clark. The winning proposal came from Bedford Associates from Bedford, Massachusetts. The result, built in 1969, was the first PLC and designated the 084, because it was Bedford Associates' eighty-fourth project. Bedford Associates started a company, Modicon, Inc., dedicated to developing, manufacturing, selling, and servicing this new product, which they named Modicon (standing for "modular digital controller"). One of the people who worked on that project was Dick Morley, who is considered to be the father of the PLC. The Modicon brand was sold in 1977 to Gould Electronics and later to Schneider Electric, its current owner. About this same time, Modicon created Modbus, a data communications protocol to be used with its PLCs. Modbus has since become a standard open protocol commonly used to connect many industrial electrical devices. One of the first Modicon 084 models built is now on display at Schneider Electric's facility in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range like Modicon Micro 84 and Modicon TSX CSY 84 until after the 984 made its appearance.

Allen-Bradley In a parallel development, Odo Josef Struger is sometimes known as the "father of the programmable logic controller" as well. He was involved in the invention of the Allen-Bradley programmable logic controller. Prior to the release of the IBM "Personal Computer" in 1981, these devices were commonly known simply as "Programmable Controllers", or "PC's. But rapid expansion of the personal computer industry in the early 1980's, the abbreviation "PC" was rapidly and universally accepted as a reference to "Personal Computers". Odo was later credited with coining the "PLC" acronym to distinguish these two technologies, And the inclusion of the word "Logic" was an easy choice as "Ladder Logic" had already been accepted as the name of the programming language. Allen-Bradley (now a brand owned by Rockwell Automation) went on to become the dominant PLC manufacturer in the United States during his tenure. Struger played a leadership role in developing IEC 61131-3 PLC programming language standards. Allen-Bradley's PLC-5 family, preceded by the 1774-PLC (released in the 1980's, was the first modular design found in modern-day systems. While Modicon was the most popular of the first full featured PLC's through the early 1980's, Allen Bradley overtook them in the mid-1980's and grew to own the largest market share of PLC hardware and software in North America.

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Illustrations

Worked examples

Example 1 — a first encounter with Programmable logic controller

Start with the simplest possible case. Write down what Programmable logic controller claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Programmable logic controller before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Programmable logic controller ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Programmable logic controller

In research: Programmable logic controller appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Programmable logic controller in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Programmable logic controller is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer engineering, Industrial automation, Industrial computing, so understanding it makes those chapters shorter.
In everyday life: Look for Programmable logic controller outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Programmable logic controller in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Programmable logic controller means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Programmable logic controller out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Programmable logic controller in simple terms?

Why does Programmable logic controller matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Programmable logic controller?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Programmable logic controller.