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A programmable logic controller PLC or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly linesor robotic devices, or any activity that requires high reliability control and grafceg of programming and process fault diagnosis. They were first developed in the automobile manufacturing industry to provide flexible, ruggedized and easily programmable controllers to replace hard-wired relays, timers and sequencers.

Since then they have been widely adopted as high-reliability automation controllers suitable for harsh environments. A PLC is an example of a “hard” real-time system since output rgafcet must be produced in response to input conditions within a limited time, otherwise unintended operation will result.

Programs to control machine grafcdt are typically stored in battery-backed-up or non-volatile memory. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relayscam timersdrum reand dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics.

When digital computers became available, being general-purpose programmable devices, they were soon applied to control sequential and combinatorial logic in industrial processes.

However these early computers required specialist programmers and stringent operating environmental control for temperature, cleanliness, and power quality.

To meet these challenges the PLC was developed with several key attributes. It would tolerate the shop-floor environment, it would support discrete bit-form input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation ed be monitored. Since many industrial processes have timescales easily addressed by millisecond response times, modern fast, small, reliable electronics greatly facilitate building reliable controllers, and performance could be traded off for reliability.

In 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 Trancue R. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the because it was Bedford Associates’ eighty-fourth project, was the result.

Trnache Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modiconwhich stood for modular digital controller. One of the people who worked on that project was Dick Morleywho is considered to be the “father” of the PLC. One of the very first models built is now on display at Schneider Electric’s facility in North Andover, Massachusetts. It was presented to Modicon by GMwhen the unit was retired after nearly twenty years of uninterrupted service.

Modicon used the 84 moniker at the grafcrt of its product range until the made its appearance. In a parallel development Odo Josef Struger is sometimes tranceh as the “father of the programmable logic controller” as well.

He was involved in the rgafcet of the Allen-Bradley programmable logic controller PLC during to Struger is credited with creating the PLC acronym. Allen-Bradley now a brand owned by Rockwell Automationthe manufacturer of the controller, became a major programmable logic controller device manufacturer in the United States during the tenure of Struger.

Early PLCs were pa to replace relay logic systems.

These PLCs were programmed in ” ladder logic “, which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians.

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Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. Another method is state logica very high-level programming language designed to program PLCs based on state tarnche diagrams. Many early PLCs did not have accompanying programming terminals that were capable of graphical representation of the logic, and so grafcft logic was instead represented as a series of logic expressions in some version of Boolean formatsimilar to Boolean algebra.


As programming terminals evolved, it became more common for ladder logic to be used, for the aforementioned reasons and because it trancbe a familiar format used for electro-mechanical control panels.

Newer formats such as state logic and Function Block which is similar to the way logic is depicted when using digital integrated logic circuits exist, but they are still not as popular as ladder logic.

A primary reason for this is that PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer the person writing the logic to see any issues with the timing of al logic sequence more easily than would be possible in other formats. PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. Often, a single PLC can be programmed to replace thousands of relays.

Early Grafcett, up to the mids, were programmed using proprietary programming panels or special-purpose programming terminalswhich often had dedicated function keys representing the various logical grwfcet of PLC programs. Some proprietary programming terminals displayed the elements of PLC ls as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity.

The oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on personal computers, which now represent the logic in graphic form instead of character symbols. The programming software allows entry and editing of the ladder-style logic.

In some software packages, it is also possible to view and edit the program in function block diagrams, sequence flow charts and structured text. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation.

The software will upload and download the PLC program, for backup geafcet restoration purposes. Garfcet some models of programmable controller, the program is transferred from a personal computer to the PLC through a programming board which writes the program into a removable chip such as an EPROM.

The most commonly used programming language is Ladder diagram LD also known as Ladder logic. It uses Contact-Coil logic to make programs like an electrical control diagram. Tranch graphical programming notation called Sequential Function Charts is available on certain programmable controllers. A model which emulated electromechanical control panel devices such as the contact and coils of relays which PLCs replaced.

This model remains common today. IEC currently defines five programming languages for programmable control systems: These techniques emphasize logical ds of operations. Even within the same product line of a single manufacturer, different models may not be directly compatible. This is a programming example in ladder diagram which shows the control system.

A ladder diagram is a method of drawing control circuits which gradcet PLCs. The ladder diagram resembles the schematic diagram of a system built with electromechanical relays. As an example, say a trancche needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank by controlling the valve that refills the tank.

In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system.

If a discrete input is energized, the memory bit is a 1, and a “normally open” contact controlled by that bit will pass a logic “true” signal on to the next element of the ladder. Therefore, the contacts in the PLC program that “read” or look at the physical switch contacts in this case must be “opposite” or open in order to return a TRUE for the closed physical switches.

Internal status bits, corresponding to the state of discrete grafcst, are also available to the program. In the example, the physical state of the float switch contacts must be gradcet when choosing “normally open” or “normally closed” symbols in the ladder diagram. Both float switches normally closed open their contacts when the water level in the tank is above the physical location of the switch.

When the water level is ttranche both switches, the float switch physical contacts are both closed, and a true logic 1 value is passed to the Fill Valve output.

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Water begins to fill the tank. The internal “Fill Valve” contact latches the circuit so that even when the “Low Level” contact opens as the water passes the lower switchthe fill valve remains on. Since the High Level is also normally closed, water continues to flow as the water level remains between the two switch levels. Once the water level rises enough so that the “High Level” switch is off openedthe PLC will shut the inlet to stop the water from overflowing; this is an example of seal-in latching logic.


The output is sealed in until a high level condition breaks the circuit.

After that the fill valve remains off until the level drops so low that the Low Level switch is activated, and the process repeats again. A complete program may contain thousands of rungs, evaluated in sequence. A complete program scan may take only a few milliseconds, much faster than changes in the controlled process. Programmable controllers vary in their capabilities for a “rung” of a ladder diagram.

Some only allow a single output bit. There are typically limits to the number of series contacts in line, and the number of branches that can be used.

Each element of the rung is evaluated sequentially.

If elements change their state during evaluation of a rung, hard-to-diagnose faults can be generated, although sometimes as above the technique is useful. Some implementations forced evaluation from left-to-right as displayed and did not allow reverse flow of a logic signal in multi-branched rungs to affect the output.

The functionality of the PLC has evolved over the years to include sequential relay control, ggafcet control, process controldistributed control systemsand networking. The data handling, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. Desktop computer controllers have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than do PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs.

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Operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the controller may not always respond to changes of input status with the consistency in timing expected from PLCs. Desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical.

The most basic function of a programmable controller is to emulate the functions of electro-mechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of “examine if on” instructions will energize its output storage bit if all the input bits are on.

Similarly, a parallel set of instructions will perform a logical OR. In an electro-mechanical relay wiring diagram, a group of contacts controlling one coil is called a “rung” of a “ladder diagram “, and this concept is also lla to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one “rung” of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an “internal coil” with no physical connection.

Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program. Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which in a sufficiently trannche circuit may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts.

The elimination of these “sneak paths” is either grafcwt bug or a feature, depending on programming style.

More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variable internally that may not correspond to discrete logic.