Editing Programmable logic controller (section)

==Functionality==
[[File:Siemens Simatic S7-416-3.jpg|thumb|upright|PLC system in a rack, left-to-right: power supply (PS), CPU, interface module (IM) and communication processor (CP)]]
[[File:PLC Control Panel.png|thumb|upright|Control panel with PLC (gray elements in the center). The unit consists of separate elements, from left to right: power supply, controller, relay units for input and output.]]

The main difference compared to most other computing devices is that PLCs are intended for and therefore tolerant of more severe environmental conditions (such as dust, moisture, heat, cold), while offering extensive [[input/output]] (I/O) to connect the PLC to [[sensor]]s and [[actuator]]s. PLC input can include simple digital elements such as [[limit switch]]es, analog variables from process sensors (such as temperature and pressure), and more complex data such as that from positioning or [[machine vision]] systems.<ref>Harms, Toni M. & Kinner, Russell H. P.E., ''Enhancing PLC Performance with Vision Systems''. 18th Annual ESD/HMI International Programmable Controllers Conference Proceedings, 1989, p. 387-399.</ref> PLC output can include elements such as indicator lamps, sirens, [[electric motor]]s, [[pneumatic]] or [[hydraulic]] cylinders, magnetic [[relay]]s, [[solenoid]]s, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a fieldbus or computer network that plugs into the PLC.<!--[[User:Kvng/RTH]]-->

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, [[process control]], [[distributed control system]]s, and [[computer network|networking]]. The data handling, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to [[desktop computer]]s. PLC-like programming combined with remote I/O hardware, allows a general-purpose desktop computer to overlap some PLCs in certain applications. Desktop computer controllers have not been generally accepted in heavy industry because desktop computers run on less stable operating systems than 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. 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.{{citation needed|date=November 2014}}

===Basic functions===
The most basic function of a programmable logic controller is to emulate the functions of electromechanical 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 electromechanical relay wiring diagram, a group of contacts controlling one coil is called a "rung" of a "ladder diagram", and this concept is also used 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 complex 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 a bug or a feature, depending on the 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 variables internally that may not correspond to discrete logic.

=== Communication ===
PLCs use built-in ports, such as [[USB]], [[Ethernet]], [[RS-232]], [[RS-485]], or [[RS-422]] to communicate with external devices (sensors, actuators) and systems ([[programming software]], [[SCADA]], [[user interface]]). Communication is carried over various industrial network protocols, like [[Modbus]], or [[EtherNet/IP]]. Many of these protocols are vendor specific.

PLCs used in larger I/O systems may have [[peer-to-peer]] (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for user interface devices such as keypads or [[Personal computer|PC]]-type workstations.

Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.

=== User interface ===
{{See also|User interface|List of human-computer interaction topics}}
[[File:Control-panel-plc.jpg|thumb|upright|Control panel with a PLC user interface for [[Thermal oxidiser|thermal oxidizer]] regulation]]

PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A [[SCADA#Human-machine interface|human-machine interface]] (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.