Programmable Logic Controllers: The Evolution of a Disruptive Technology
Arnold Lander posted on March 11, 2019 |

In 1975 draftsmen were drawing electrical schematics with pencils on pieces of paper taped onto drafting boards using T-squares, 45s and 30-60s as their tools. The old-timers were still using protractors to draw circles, but the young guns were using circle templates—you got the same sized circle every time, and it didn’t leave a hole in the page.

At that time, engineering was a slow, sedate business. It would be another six years before personal computers entered most engineering offices and two decades before the last drafting board fell into disuse. But in 1975 the invasion of North America's manufacturing floors by a new disruptive technology was already well underway.

New Programmable Logic Controllers (PLCs) were rapidly replacing electromechanical relays inside control panels. Early PLCs were essentially just relay replacers. They lacked power and speed, but their impact on the shop floor was profound. Control panels were downsized, complex wiring was eliminated, and maintenance was drastically reduced.

PLC Pioneers, Richard Morley, Tom Bossevain, George Schwenk and Jonas Landau,
PLC Pioneers Richard Morley, Tom Bossevain, George Schwenk and Jonas Landau

Development of what can be called a PLC began in 1968, when General Motors created a ‘Standard Machine Controller’ specification and distributed it to vendors for quote. The major elements of the spec were as follows:

  • Using solid state components unit would be modular and expandable
  • Have 16 inputs, expandable to 256
  • Have 16 outputs, expandable to 128
  • Easily programmed or reprogrammed
  • Stored programs would not be lost in power outagesHave at least 1k of memory, expandable to 4k.

Working independently, Richard E. Morley of Bedford Associates had already designed a Modular Digital Controller. Bedford’s device met all the criteria for the Standard Machine Controller and in testing on GM‘s floor showed a 60% reduction in downtime versus relay-controlled equipment. In 1969, Bedford Associates changed their name to Modicon PLC and began production of the Modicon 084.

What set the Modicon 084 apart from its competitors was its programming method. Other contenders were using “Boolean Statements” to control their equipment. This was fine for computer scientists However plant engineers and electricians found these programs hard to understand. The relay control systems they were used to employed ladder diagrams, so named because the relay circuits, drawn between a hot and neutral common, resembled the rungs of a ladder.

A ladder diagram is a schematic representation of a relay control circuit. Ladder diagrams gained popularity in the early days of automation because of their simplicity and intuitive nature.
A ladder diagram is a schematic representation of a relay control circuit. Ladder diagrams gained popularity in the early days of automation because of their simplicity and intuitive nature.

Morley felt that by mimicking the ladder diagram, his “Ladder Logic” would be much easier for engineers and maintenance personnel to understand than the Boolean logic provided by his competitors. Over time other programming methods such as instruction list, flow chart, and structured text have been developed; however, ladder logic is still a popular choice because of its graphical and intuitive nature.

Ladder logic to control a transfer bar looks very much like a relay ladder diagram but instead of coils and contacts connected by wires mounted on a backplate they are bits in a computer.
Ladder logic to control a transfer bar looks very much like a relay ladder diagram but instead of coils and contacts connected by wires mounted on a backplate they are bits in a computer.

By 1979, Allen-Bradley and others had entered the market in direct competition with Modicon. Innovation became essential to support market share. Not only were PLCs rapidly growing faster and more powerful, but programming and documentation tools were also evolving quickly. Early PLCs did not offer a method of program documentation. Programs were hand drawn on a drafting board to be “keyed in” later.

Allen Bradley 1770-T2 PLC 2 Programming Terminal circa 1980. They stuck a handle on it and called it portable. It wasn’t.

Allen Bradley 1770-T2 PLC 2 Programming Terminal circa 1980. They stuck a handle on it and called it portable. It wasn’t.

The development of Modbus by Modicon and Data Highway by Allen-Bradley made it possible for PLCs to communicate with other processors. Programming terminals were developed that would allow programmers to enter logic programs remotely. The finished program could then be copied onto a cassette tape to be downloaded to the PLC. Printouts could also be generated, eliminating the need for time-consuming hand drawings on the drafting board.

In the ‘80s, the first personal computers entered the design office. These machines were slow by today's standards but much faster than hand drawing. By the end of the decade, most electrical designers were weaned off of their drafting boards and were working exclusively on desktop computers. Lettering guides and circle templates had been relegated to the bottom drawer.

Portable computers were making their way onto the shop floor as well. Computers could now interface directly with PLCs and software improvements made monitoring machine motions much easier. The PLC program had become the best diagnostic tool available for troubleshooting and was viewed by many as a window into the machine.

Allen Bradley 1770-SA Digital Tape Cartridge Recorder. Early PLC programs were backed up on data tape cartridges.

Allen Bradley 1770-SA Digital Tape Cartridge Recorder. Early PLC programs were backed up on data tape cartridges.

Machine diagnostics were primitive in the early years, and were limited to machine maps that indicated the current condition of each station on the machine and lighted pushbuttons that showed the current status of each motion, e.g., “Clamps Closed,” “Transfer Bar Raised” and so on. From these small bits of information, maintenance electricians needed to diagnose problems and return the machine to automatic mode as quickly as possible.

In the ‘90s, end users were requesting industrial terminals with PLC monitoring software to be supplied with all new machinery. Production managers wanted maintenance personnel troubleshooting, not waiting for laptops to start up. Programs were designed for maximum readability and simplicity, rather than efficiency. 

Production managers of the day were needy folks who were easily aroused. Nothing stirred their anger more than watching an electrician scan the logic for several minutes only to find that the machine had stopped because the emergency stop pushbutton, six inches from the electricians’ right hand, had been pressed. They wanted machines that would tell them what was wrong, and the programmable human machine interface (HMI) provided a solution. The first HMIs were simple pushbutton replacers, and were considered uneconomical for applications with fewer than twenty pushbuttons. They quickly grew in popularity as manufacturers found more uses for them. As fast as PLCs and HMIs could increase their memory capacities, manufacturers were finding things to fill them up with.

Programmable Human Machine Interface. Early versions were simple Pushbutton replacers

Programmable Human Machine Interface. Early versions were simple Pushbutton replacers

Machine monitoring information was growing in importance. Machine faults, manual interventions, time in auto, production counts and more were all being tracked and displayed on HMI screens and transmitted to plant central computers. By the end of the ‘90s, the logic controlling machine motions was only a fraction of the total PLC program. HMIs were packed with so much information that maintenance workers rarely had to view the program logic.

The end of the 20th century saw the next generation of PLCs introduced which would eventually bring the internet to the plant floor. Processor memory was now measured in megabytes. User-defined data types, (UDTs) allowed machine data to be manipulated and shared in limitless ways, and connectivity to other processes had never been easier.

With the introduction of safety PLCs any resemblance to the control systems built in 1975 has now vanished. In 1975, the main panel was filled with relays, motor starters, fuses, and circuit breakers. The PLC chassis, if it existed, was filled with I/O cards. All machine inputs and outputs were wired back to the main panel via individual wires.

Today, the main panel is basically a just a PLC in a box. Instead of I/O cards, the PLC chassis is filled with communication cards. All of the machine I/O is connected to IP67-rated modules mounted on the machine. Laptops can access the logic from anywhere with an ethernet connection. Even the HMI has been moved to its own enclosure.

So, are we better off now than in 1975? Automobiles are certainly better. Modern vehicles are safer, more fuel efficient and more reliable than ever. However, designers will tell you that any time there has been an advancement that has allowed them to work faster they have ended up with more work to do, less time to do it and fewer people to help. Drafting boards are gone, draftsmen are gone, and the Chief Draftsman is probably delivering pizzas somewhere. I guess it’s a toss-up.

Link to PLC historical timeline.








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