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PROGRAMMABLE LOGIC CONTROLLERPLCs may need to interact with people for the purpose of configuration, alarm reporting or everydaycontrol. A Human-Machine Interface (HMI) is employed for this purpose.

A simple system may use buttons and lights to interact with the user. Text displays are available as

well as graphical touch screens. Most modern PLCs can communicate over a network to some othersystem, such as a computer running SCADA system or web browser.

The PLC was invented in response to the needs of the American automotive industry. Before the PLC,control, sequencing, and safety interlock logic for manufacturing automobiles and trucks wasaccomplished using relays, timers and dedicated closed-loop controllers. The process for updating

such facilities for the yearly model change-over was very time consuming and expensive, as the relaysystems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatictransmission division of General Motors) issued a request for proposal for an electronic replacementfor hard-wired relay systems.

The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC,designated the 084 because it was Bedford Associates eighty-fourth project, was the result. BedfordAssociates started a new company dedicated to developing, manufacturing, selling, and servicing thisnew product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked onthat project was Dick Morley, who is considered to be the "father" of the PLC. The Modicon brand wassold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by SchneiderElectric, the current owner.

One of the very first 084 models built is now on display at Modicon's headquarters in North Andover,Massachusetts. It was presented to Modicon by GM, when the unit was retired from nearly twentyyears of uninterrupted service.

WHAT IS PLC?

A programmable logic controller, PLC, or programmable controller is a small computer used forautomation of real-world processes, such as control of machinery on factory assembly lines. The PLC

usually uses a microprocessor. The program can often control complex sequencing and is written byskilled engineers. The program is stored in battery-backed memory and/or EEPROMs.The main difference from other computers are the special input/output arrangements. These connect

the PLC to sensors and actuators. PLCs read limit switches, dual-level devices, temperature indicatorsand the positions of complex positioning systems. Some even use machine vision. On the actuatorside, PLCs drive any kind of electric motor, pneumatic or hydraulic cylinders or diaphragms, magneticrelays or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC mayhave external I/O modules attached to a proprietary computer network that plugs into the PLC.

PLCs were invented as less expensive replacements for older automated systems that would usehundreds or thousands of relays and cam timers. Often, a single PLC can be programmed to replacethousands of relays. Programmable controllers were initially adopted by the automotive manufacturing

industry, where software revision replaced the re-wiring of hard-wired control panels.

The functionality of the PLC has evolved over the years to include typical relay control, sophisticatedmotion control, process control, Distributed Control Systems and complex networking.

The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electricalconnection diagrams. The electricians were quite able to trace out circuit problems with schematicdiagrams using ladder logic. This was chosen mainly to reduce the apprehension of the existingtechnicians.

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Today, the line between a programmable computer and a PLC is thinning. The PLC has been provenvery reliable, but the programmable computer still has a ways to go. the With the IEC 61131-3standard, it is now possible to program these devices using structured programming languages, andlogic elementary operations. A graphical programming notation called Sequential Function Charts isavailable on certain programmable controllers.

However, it should be noted that PLCs no longer have a very high cost (often thousands of dollars),typical of a "generic" solution. Modern PLC's with full capabilites are availble for a few hundred USD.There are other ways for automating machines, such as a custom microcontroller-based design, butthere are differences among both: PLCs contain everything needed to handle high power loads rightout of the box, while a microcontroller would need an electronics engineer to design power supplies,power modules, etc. Also a microcontroller based design would not have the flexibility of in-fieldprogrammability of a PLC. That is why PLCs are used in production lines, for example. These typicallyare highly customized systems so the cost of a PLC is low compared to the cost of contacting adesigner for a specific, one-time only design. On the other hand, in the case of mass-produced goods,customized control systems quickly pay for themselves due to the lower cost of the components.

PROGRAMMABLE LOGIC CONTROLLER SOFTWARE

PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday

control. A Human-Machine Interface (HMI) is employed for this purpose.A simple system may use buttons and lights to interact with the user. Text displays are available as

well as graphical touch screens. Most modern PLCs can communicate over a network to some othersystem, such as a computer running SCADA system or web browser.

The PLC was invented in response to the needs of the American automotive industry. Before the PLC,control, sequencing, and safety interlock logic for manufacturing automobiles and trucks wasaccomplished using relays, timers and dedicated closed-loop controllers. The process for updating

such facilities for the yearly model change-over was very time consuming and expensive, as the relaysystems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatictransmission division of General Motors) issued a request for proposal for an electronic replacementfor hard-wired relay systems.

The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC,designated the 084 because it was Bedford Associates eighty-fourth project, was the result. BedfordAssociates started a new company dedicated to developing, manufacturing, selling, and servicing thisnew product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked onthat project was Dick Morley, who is considered to be the "father" of the PLC. The Modicon brand wassold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by SchneiderElectric, the current owner.

One of the very first 084 models built is now on display at Modicon's headquarters in North Andover,Massachusetts. It was presented to Modicon by GM, when the unit was retired from nearly twentyyears of uninterrupted service.

PLC Vendors

The PLC was invented in response to the needs of the American automotive industry. Before the PLC,control, sequencing, and safety interlock logic for manufacturing automobiles and trucks wasaccomplished using relays, timers and dedicated closed-loop controllers. The process for updatingsuch facilities for the yearly model change-over was very time consuming and expensive, as the relaysystems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatictransmission division of General Motors) issued a request for proposal for an electronic replacementfor hard-wired relay systems.

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The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of itscontroller models such that they end with eighty-four. PLCs are used in many different industries andmachines such as packaging and semiconductor machines. Well known PLC brands are Honeywell,Siemens, Schneider Electric, Omron, Rockwell (Allen Bradley), General Electric, Panasonic(Matsushita) and Mitsubishi.