PLC Programming Then & Now: The History of PLC’s

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PLC Programming Then & Now: The History of PLC’s

The first PLC was introduced in the late 1960s.

A programmable logic controller is an industrial computer. It is designed to help in the control of manufacturing processes.

The automobile sector was the first industry to deploy PLCs into its operations. Their aim was to replace the hardwired relays and timers with programmable and flexible controllers.

Since then, PLCs have been broadly adopted as the standard automation control system in discrete manufacturing industries.

This article will discuss the history of PLC programming and how it has evolved over the years.

Inception of PLC Programming

The first PLC began being developed in 1968. General Motors designed a specification for a ‘Standard Machine Controller’ and distributed it to vendors for a quote. Some of the major elements of the specification included:

  • Should use solid state components which should be modular and expandable
  • Should contain 16 units that can be expanded to 256
  • Should have 16 outputs that can be expanded to 128
  • Should offer easy programming and reprogramming
  • Should not lose stored programs during power outages, therefore, have at least 1k of memory that can be expanded to 4k

Richard E. Morley, who worked for Bedford associates, designed a device known as the Modular Digital Controller.

This device met all the requirements the Standard Machine Controller was asking for. When the Modular Controller was tested in General Motors, it showed a 60 percent reduction in downtime.

Modicon PLC

Following this success, Bedford Associates changed its name to Modicon PLC. They began producing the Modicon 084, the first PLC.

What differentiated the Modicon 084 from other products in the market was its programming technique. The others were utilizing ‘Boolean Statements’ to manipulate their equipment.

Boolean algebra was by the Irish Mathematician George Boole and presented in The Mathematical Analysis of Logic (1847). Boolean mathematics is the math of ones and zeros, True and False. At it’s core, it consists of three expressions, AND, OR and NOT. All computers use this type of logic.

Despite the simplicity of Boolean Logic and The Genius of George Boole, Boolean Statement programs were okay for computer scientists. However, plant engineers found them difficult to work with, as compared to relay logic.

The engineers were used to relay control systems which employed ladder diagrams. This is because whenever relay circuits are drawn between a hot and neutral common, they resemble the rungs of a ladder.

Morley’s genius idea was to incorporate ‘ladder logic’ into his system. Ladder Logic is essentially a graphic representation of Boolean Logic. This was the game changer. The engineers would find it easier to understand and use than Boolean Logic. Example of how Boolean can be expressed as Ladder Logic. (Source:

Process Description

A heating oven with two bays can heat one ingot in each back. When the heater is on it provides enough heat for two ingots. But, if only one ingot is present the oven may become too hot, so a fan is used to cool the oven when it passes a set temperature.

Control Description

If the temperature is too high AND there is an ingot in only one bay, turn the fan on.

Define Inputs and Outputs
B1 = Bay 1 ingot present
B2 = Bay 2 ingot present
F = Fan
T = Temperature sensor

By the time the 70s were coming to a close, Allen-Bradley and other competitors had developed systems that rivaled Modicon. Innovation started becoming key to capturing market share.

PLCs were becoming faster and more powerful. There was also the rapid evolving of programming and documentation tools.

The initial PLCs did not have a platform for program documentation. Thus, a program had to be hand-written or drawn on a drafting board before it was entered later.

This is the period that saw to the development of the Data Highway by Allen-Bradley and Modbus by Modicon. These innovations allowed PLCs to exchange information with each other.

There was also the development of programming terminals. These allowed programmers to remotely enter logic programs. The final program could then be recorded on cassette tape then downloaded to a PLC.

They could also generate printouts. This eliminated the need for the hand-drawings that consumed a lot of time.

The 1980s

The ‘80s saw the introduction of the first personal computers in offices. While you can’t compare their speed to today’s computers, they were still a lot faster than drawing on drafting boards.

In fact, nearly every designer had replaced their drafting board with a desktop computer by the time the 1980s were coming to a close.

The adoption of personal computers was not relegated just to the design arena, but also the shop floor.

The PCs began being used to interface with PLCs directly. Coupled with software improvements, this made monitoring machine motions a lot easier.

By this period, the PLC program was widely recognized as the most useful diagnostic tool. It allowed for effective troubleshooting, thus many considering it to be the window to machines.

Nonetheless, machine diagnostics were still in their primitive stages.

More powerful programing languages evolve
As PLC’s evolved, other programming languages were developed. These include flow charts, structured text, and instruction list. Nonetheless, ladder logic remains popular due to its graphical and intuitive design.

The Late 70s

Development of the IEC 61131-3
One of the most significant milestones in PLC history was the introduction of the International Electrotechnical Commission (IEC) 61131-3 specification in 1982.

It was the standard by which PLC software being developed was to be held against. It became published in 1993 as IEC 1131 International Standard for Programmable Controllers.

The introduction of the IEC 61131-3 was necessary as it brought consistency to all the software products on the market. This allows engineers and technicians to easily understand logic and program flow from any PLC software.

The 1990s

As the ‘90s rolled in, end users began making special requests. They wanted their new machinery to come with industrial terminals that had PLC monitoring software.

Plant managers wanted their technicians doing actual troubleshooting. As such, the PLC programs at that time were simple in design.

In an attempt to save time, plant managers wanted to have machines that could tell them what was amiss. Instead of spending hours troubleshooting.

This is what led to the development of the programmable human-machine interface (HMI).

Programmable Human Machine Interface

The prototype HMIs were modest pushbutton replacers. However, they were thought to be uneconomical for applications that had less than 20 pushbuttons.

Nonetheless, their popularity started growing as manufacturers began finding more uses for them.

Machine monitoring information was becoming more and more vital.

This included information such as machine problems, time in auto, manual interventions, production counts, and more. All that was being monitored and displayed in HMI screens then later sent to the factory’s central computers.

As the 1990s drew to an end, the logic controlling functions were just a slight part of what a PLC program could do. This is because HMIs had so much data that technicians hardly looked at program logic.

The end of the ‘90s saw the introduction of a new generation of PLCs. These new devices are what eventually brought internet connectivity to the factory floor.

Megabytes became the new standard for measuring processor memory. There was also the introduction of User-defined data types. These allowed for the manipulation and sharing of machine data in many ways.

PLCs Today

Since the beginning, there has always been a need to reduce the size of the automation systems so as to make support and maintenance simpler. This is why we are witnessing the following trends in PLC technology:

Better, Smaller, and Faster
Circuit boards, processors, and other electronic components are rapidly shrinking. These improvements are influencing how PLCs are being designed.

However, some things are affecting the acceptance of these changes. These include the need for ruggedness, reliability, and stability.

Thus, the most prevalent enhancement in the PLC industry is speed. This is enabled by faster processors. They improve cycle time, have new communication features, and enhanced memory capacity.

As the market continues to make demands, a lot of functions and features traditionally allocated to high-end PLCs are making their way to the lower-end products.

You can, therefore, expect to see smaller PLCs spotting features associated with top-tier machines. This allows for a smaller and more compact solution that today’s users desire.

Memory Size
Today’s PLCs are also taking advantage of the dramatically declining sizes and costs of solid-state memory.

This is allowing for enhanced local data storage. This enables PLCs to be used in a lot of applications that traditionally required expensive data acquisition systems.

This feature also allows for additional utilities. This includes the ability to store information on-board, thus expediting troubleshooting.

Memory Devices
Another technology that is making its way to the industrial controls market are portable memory devices. These devices are highly beneficial to the PLC user.

They provide you with massive amounts of extra storage in small packages. For example, a microSD card can add up to 32 GB of extra memory to the PLC.

The Merging of PLCs and PACs

Programmable automation controllers (PACs) are hardened modular industrial controllers. They often utilize a PC-based processor. This allows them to have more flexibility and depth in programming.

For a long time, suppliers in the industrial controls industry have been citing the differences between PACs and PLCs.

However, as PLC technology evolves, automation engineers cease to care about their differences. Instead, they will start focusing on the performance and available features. This will allow them to define their systems better.

PACs and PLCs will continue merging as they evolve. As that happens, there are bound to be opportunities in the market for both high and low-end processors.

As hardware technology continues to evolve, advanced features will start being incorporated into low-tier processors as well.

As a result, manufacturers of the higher-end devices will be forced to include even more options and features into their products.

Advanced Features

As processors become faster and memories became even larger. It allowed a floodgate of advanced features into the market. These included vision system integration, motion control, as well as synchronized support for multiple communication protocols. All that while maintaining the simplistic nature that makes the PLC ideal to most consumers.

In the period that PLC and PAC have been going head to head, we have witnessed a more rapid development of both products.

PACs are allowing their consumers to test the limits of what is regarded as traditional industrial automation. This is forcing PACs manufacturers to develop products that can meet those demands.

Thus, product designers are having to come up with innovative designs. These sustain the available components and build them into rugged systems. This allows them to withstand the harsh industrial environment.

Ladder Logic Is Here to Stay

As mentioned earlier, about half-a-century ago, the ladder diagram replaced hardwired relay logic.

While ladder logic made things simpler for engineers and technicians, it has some drawbacks. Most notably, it is not efficient in data handling and process control.

This saw to the development of other industrial controller programming languages by the IEC 6113. The standard covers the following:

  • Ladder Logic
  • Structured Text
  • Function Block
  • Instruction List
  • Sequential Function Charts

For instance, sequential flow charts are better for process control. Structured text is good for data manipulation.

Other languages have their strong points as well. Nonetheless, the ladder diagram has stayed on course through various advancements. It remains the most desired language in PLC programming languages.

Connected Factory

The most impactful change expected in the future of PLCs is their integration with Enterprise Resource Planning. As well as synchronizing with other high-level computing systems to the factory premises.

In the past, extracting data and feeding it upstream to those systems was a major integration task. However, future technologies are expected to have features, functions, and hooks that allow for simplified integration.

The Rise of Industrial Ethernet and Industrial Internet of Things
The industrial connectivity landscape has witnessed major changes. Industrial Ethernet is now the network of choice on the factory floor.

An industrial Ethernet network can handle larger amounts of data, and at faster speeds. This makes it ideal for high-end and data-heavy applications that a PLC typically contains.

Another key reason for the adoption of industrial Ethernet is the recent development of the Industrial Internet of Things.

IoT allows manufacturers to connect all their equipment so they behave as a single module. This is made possible by connectors and sensors.

They are fitted to PLCs and other industrial devices to make data gathering more efficient. This gives managers an excellent real-time view of what’s happening on the floor, thus ensuring trouble areas are spotted as soon as they surface.

User Needs

Controller suppliers will have to take customers’ needs into consideration. This means coming up with new PLC solutions.

The PLC is likely to remain the gold standard for automation controllers. Nonetheless, a lot of changes might take place in PLC programming that will enhance its purpose and performance.

Thus, you can expect PLCs to become smaller, contain more features, and continue to be rugged. Essentially, it will be a totally new industrial automation system with an old name.

Are you looking to learn more about how to utilize PLCs in your industrial functions?

c3controls is the leader in industrial innovations. We have been creating systems that make industrial processes efficient and cost-effective for a long time. Reach out to us today to learn how we can be of help to your enterprise.

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