Editing Programmable logic controller (section)

==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.<ref>{{Harvnb|Parr|1998|p=438}}</ref> Before, control logic for manufacturing was mainly composed of [[relay]]s, [[cam timer]]s, [[Drum sequencer (controller)|drum sequencers]], and dedicated [[closed-loop controller]]s.<ref>{{cite web |last=Wayand |first=Ben |url=https://www.mroelectric.com/blog/what-is-a-plc/ |title=What Is a PLC? |website=MROElectric.com |date=20 March 2020 |publisher=MRO Electric |access-date=11 May 2021 }}</ref>

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.<ref>{{Cite web |url=https://www.controlsystemsandautomation.com/learn/plc/plc-programming-basics-i/ |title=PLC Programming Basics Part I |date=2019-07-23 |website=Control Systems & Automation |access-date=2020-02-23 }}</ref> When general-purpose computers became available, they were soon applied to control logic in industrial processes. These early computers were unreliable<ref>{{Harvnb|Laughton|Warne|2002|p=16/3}}: "The first industrial computer application was probably a system installed in an oil refinery in Port Arthur USA in 1959. The reliability and mean time between failure of computers meant that little actual control was performed."</ref> and required specialist programmers and strict control of working conditions, such as temperature, cleanliness, and power quality.<ref>{{Harvnb|Parr|1998|p=437}}</ref>

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 language]]s. Early PLCs were programmed in [[ladder logic]], which strongly resembled a schematic diagram of [[relay logic]]. It also permitted its operation to be monitored.<ref>{{Harvnb|Bolton|2015|p=6}}</ref><ref>{{Harvnb|Parr|1998|pp=438, 450–451}}</ref>

===Modicon===
In 1968, GM Hydramatic,<!--Don't link hydramatic transmission--> the [[automatic transmission]] division of [[General Motors]], issued a [[request for proposal]]s 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.<ref name=":9">{{Harvnb|Laughton|Warne|2002|loc=chpt. 16}}</ref><ref name=":0">{{Cite web |url=https://www.automationmag.com/855-the-father-of-invention-dick-morley-looks-back-on-the-40th-anniversary-of-the-plc/ |title=The Father of Invention: Dick Morley Looks Back on the 40th Anniversary of the PLC |last=Dunn |first=Alison |date=2009-06-12 |website=Manufacturing Automation |access-date=2020-02-23 }}</ref>

Bedford Associates started a company dedicated to developing, manufacturing, selling, and servicing this new product, which they named '''{{visible anchor|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.<ref name=":1">{{Cite web|url=https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2003/august/cover-story-50th-anniversary-leaders-of-the-pack/|title=Leaders of the pack|last=Strothman|first=Jim|date=2003-08-01|website=ISA|url-status=live|archive-url=https://web.archive.org/web/20170808184918/https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2003/august/cover-story-50th-anniversary-leaders-of-the-pack/|archive-date=2017-08-08|access-date=2020-02-24}}</ref> The Modicon brand was sold in 1977 to [[Gould Electronics]] and later to [[Schneider Electric]], its current owner.<ref name=":0" /> About this same time, Modicon created [[Modbus]], a data communications protocol used with its PLCs.  Modbus has since become a standard open protocol commonly used to connect many industrial electrical devices.<ref>{{cite web |title=Mobus Networking Guide: Introduction |url=https://development.libelium.com/modbus_networking_guide/introduction |website=Libelium.com |access-date=27 October 2022 }}</ref>

One of the first 084 models built is now on display at Schneider Electric's facility in [[North Andover, Massachusetts]]. It was presented to Modicon by [[General Motors|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 until after the 984 made its appearance.<ref>{{cite book |last=Chakraborty |first=Kunal |title=Industrial Applications of Programmable Logic Controllers and SCADA |date=2016 |publisher=Anchor Academic Publishing |location=Hamburg |isbn=978-3960670247 }}</ref>

===Allen-Bradley===
In a parallel development, Odo Josef Struger is sometimes known as the "father of the programmable logic controller" as well.<ref name=":1" /> He was involved in the invention of the [[Allen-Bradley]] programmable logic controller<ref name=":2">{{Cite web |url=https://www.controleng.com/articles/a-b-plc-inventor-dr-odo-struger-dies/ |title=A-B PLC Inventor, Dr. Odo Struger, Dies |date=1999-02-01 |website=Control Engineering |url-status=live |archive-url=https://web.archive.org/web/20200224210429/https://www.controleng.com/articles/a-b-plc-inventor-dr-odo-struger-dies/ |archive-date=2020-02-24 |access-date=2020-02-24 }}</ref><ref name="nytimes2">{{cite news |last=Brier |first=Steven E. |url=https://query.nytimes.com/gst/fullpage.html?res=9D00E6DF173FF934A15751C1A96E958260 |title=O. Struger, 67, A Pioneer In Automation |date=1998-12-27 |newspaper=The New York Times |access-date=2020-02-24 |quote=Dr. Odo J. Struger, who invented the programmable logic controller, which makes possible modern factory automation, amusement park rides and lavish stage effects in Broadway productions, died on December 8 in Cleveland. He was 67. }}</ref><ref name="anzovin2">Anzovin, p. 100, item # 2189. ''Programmable logic controller was invented by the Austrian-born American engineer Odo J. Struger in 1958–60 at the Allen-Bradley company in Milwaukee, WI, USA. A programmable logic controller, or PLC, is a simple electronic device that allows precise numerical control of machinery. It is widely used to control everything from washing machines to roller coaster to automated manufacturing equipment.''</ref> and is credited with coining the PLC acronym.<ref name=":1" /><ref name=":2" /> Allen-Bradley (now a brand owned by [[Rockwell Automation]]) became a major PLC manufacturer in the United States during his tenure.<ref name="short2">{{cite web |url=http://www.jimpinto.com/writings/automationhistory.html |title=A Short History of Automation Growth |access-date=2008-06-20 }}</ref> Struger played a leadership role in developing [[IEC 61131-3]] PLC programming language standards.<ref name=":1" />

===Early methods of programming===
Many early PLC programming applications were not capable of graphical representation of the logic, and so it was instead represented as a series of logic expressions in some kind of Boolean format, similar to [[Boolean algebra]]. As programming terminals evolved, because ladder logic was a familiar format used for electro-mechanical control panels, it became more commonly used. Newer formats, such as state logic,<ref>{{cite web |url=https://control.com/technical-articles/state-machine-programming-in-ladder-logic/ |title=State Machine Programming in Ladder Logic |access-date=2024-08-18}}</ref> [[function block diagram]]s, and [[structured text]] exist. Ladder logic remains popular because PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the person writing the logic to see any issues with the timing of the logic sequence more easily than would be possible in other formats.<ref>{{cite web |title=Wrapping Your Head around Ladder Logic |date=27 August 2018 |url=https://www.dosupply.com/tech/2018/08/27/wrapping-your-head-around-ladder-logic/ |website=DoSupply.com |access-date=19 October 2020}}</ref>

Up to the mid-1990s, PLCs were programmed using proprietary programming panels or special-purpose programming [[Computer terminal|terminals]], which often had dedicated function keys representing the various logical elements of PLC programs.<ref name=":9" /> Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain [[ASCII art|ASCII]] character representations of contacts, coils, and wires were common. Programs were stored on [[cassette tape cartridge]]s. Facilities for printing and documentation were minimal due to a lack of memory capacity. The oldest PLCs used [[magnetic-core memory]].{{citation needed|date=May 2025}}