Editing Programmable logic controller (section)

==Architecture==
A PLC is an industrial microprocessor-based controller with programmable memory used to store program instructions and various functions.<ref>{{Harvnb|Bolton|2015|p=5}}</ref> It consists of:
* A processor unit (CPU) which interprets inputs, executes the control program stored in memory and sends output signals,
* A power supply unit which converts AC voltage to DC,
* A memory unit storing data from inputs and program to be executed by the processor,
* An input and output interface, where the controller receives and sends data from and to external devices,
* A communications interface to receive and transmit data on communication networks from and to remote PLCs.<ref name=":4">{{Harvnb|Bolton|2015|p=7}}</ref>

PLCs require a programming device which is used to develop and later download the created program into the memory of the controller.<ref name=":4" />

Modern PLCs generally contain a [[real-time operating system]], such as [[OS-9]] or [[VxWorks]].<ref name=":5" />

===Mechanical design===
[[File:Siemens sps logo 8 12-24 RCE-03.jpg|thumb|Compact PLC with 8 inputs and 4 outputs]]
[[File:PLC AB InstaladoV1.JPG|alt=Modular PLC with EtherNet/IP module, digital and analog I/O, with some slots being empty.|thumb|Modular PLC with [[EtherNet/IP]] module, discrete and analog I/O, with some slots being empty]]

There are two types of mechanical design for PLC systems. A ''single box'' (also called a ''brick'') is a small programmable controller that fits all units and interfaces into one compact casing, although, typically, additional expansion modules for inputs and outputs are available. The second design type{{snd}} a ''modular'' PLC{{snd}} has a chassis (also called a ''rack'') that provides space for modules with different functions, such as power supply, processor, selection of I/O modules and communication interfaces{{snd}} which all can be customized for the particular application.<ref>{{Harvnb|Bolton|2015|pp=12–13}}</ref> Several racks can be administered by a single processor and may have thousands of inputs and outputs. Either a special high-speed serial I/O link or comparable communication method is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.{{Citation needed|date=April 2020}}
===Discrete and analog signals===
[[Digital signal|Discrete (digital) signals]] can only take ''on'' or ''off'' value (1 or 0, ''true'' or ''false''). Examples of devices providing a discrete signal include [[limit switch]]es and [[photoelectric sensor]]s.<ref name=":8">{{Harvnb|Bolton|2015|pp=23–43}}</ref>

[[Analog signal]]s can use voltage or current that is analogous to the monitored variable and can take any value within their scale. Pressure, temperature, flow, and weight are often represented by analog signals. These are typically interpreted as integer values with various ranges of accuracy depending on the device and the number of bits available to store the data.<ref name=":8" /> For example, an analog 0 to 10&nbsp;V or 4-20&nbsp;mA [[current loop]] input would be [[analog-to-digital converter|converted]] into an integer value of 0 to 32,767. The PLC will take this value and translate it into the desired units of the process so the operator or program can read it.

===Redundancy===
Some special processes need to work permanently with minimum unwanted downtime. Therefore, it is necessary to design a system that is [[fault tolerant]]. In such cases, to increase the system availability in the event of hardware component failure, [[Redundancy (engineering)|redundant]] CPU or I/O modules with the same functionality can be added to a hardware configuration to prevent a total or partial [[Plant process and emergency shutdown systems|process shutdown]] due to hardware failure.  Other redundancy scenarios could be related to safety-critical processes, for example, large hydraulic presses could require that two PLCs turn on output before the press can come down in case one PLC does not behave properly.