Editing Programmable logic controller (section)

==PLC compared with other control systems==
[[File:BMA Automation Allen Bradley PLC 3.JPG|thumb|PLC installed in a control panel]]
[[File:Control-panel.jpg|thumb|Control center with a PLC for a [[regenerative thermal oxidiser|RTO]] ]]

PLCs are well adapted to a range of [[automation]] tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems, so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economical. This is due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands or millions of units.{{Citation needed|date=February 2020}}

Programmable controllers are widely used in motion, positioning, or torque control. Some manufacturers produce motion control units to be integrated with PLC so that [[G-code]] (involving a [[CNC]] machine) can be used to instruct machine movements.{{Citation needed|date=July 2009}}

=== PLC chip / embedded controller ===
These are for small machines and systems with low or medium volume. They can execute PLC languages such as Ladder, Flow-Chart/Grafcet, etc. They are similar to traditional PLCs, but their small size allows developers to design them into custom printed circuit boards like a microcontroller, without computer programming knowledge, but with a language that is easy to use, modify and maintain. They sit between the classic PLC / micro-PLC and microcontrollers.{{Citation needed|date=September 2023}}

=== Microcontrollers ===
A [[microcontroller]]-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware, and necessary testing and certification) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomical.<ref name="McMillan 1999">{{cite book |first=Gregory K. |last=McMillan |editor-first=Douglas M. |editor-last=Considine |title=Process/Industrial Instruments and Controls Handbook |edition=Fifth |publisher=McGraw-Hill |date=1999 |isbn=0-07-012582-1 |chapter=Section 3: Controllers }}</ref>

=== Single-board computers ===
Very complex process control, such as those used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. [[Single-board computer]]s using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a version of commercial operating systems adapted for process control needs.<ref name="McMillan 1999" />

The rising popularity of [[Single-board computer|single board computers]] has also had an influence on the development of PLCs. Traditional PLCs are generally [[closed platform]]s, but some newer PLCs (e.g. groov EPIC from [[Opto 22]], ctrlX from [[Bosch Rexroth]], PFC200 from [[WAGO Kontakttechnik|Wago]], PLCnext from [[Phoenix Contact]], and Revolution Pi from Kunbus) provide the features of traditional PLCs on an [[open platform]].

===Programmable logic relays (PLR)===
{{Original research section|date=March 2020}}

In more recent years,{{When|date=February 2020}} small products called programmable logic relays (PLRs) or smart relays, have become more common and accepted. These are similar to PLCs and are used in light industries where only a few points of I/O are needed, and low cost is desired. These small devices are typically made in a common physical size and shape by several manufacturers and branded by the makers of larger PLCs to fill their low-end product range. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Most such devices include a tiny [[postage stamp]]-sized LCD screen for viewing simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus four more separate push-buttons, similar to the key buttons on a [[VCR]] remote control, and used to navigate and edit the logic. Most have an [[RS-232]] or [[RS-485]] port for connecting to a PC so that programmers can use user-friendly software for programming instead of the small LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, but their cost can be significantly lower than that a PLC, and they still offer robust design and deterministic execution of the logic.

A variant of PLCs, used in remote locations is the [[remote terminal unit]] or RTU. An RTU is typically a low power, ruggedized PLC whose key function is to manage the communications links between the site and the central control system (typically [[SCADA]]) or in some modern systems, "The Cloud". Unlike factory automation using wired communication protocols such as [[Ethernet]], communications links to remote sites are often radio-based and are less reliable. To account for the reduced reliability, RTU will buffer messages or switch to alternate communications paths. When buffering messages, the RTU will timestamp each message so that a full history of site events can be reconstructed. RTUs, being PLCs, have a wide range of I/O and are fully programmable, typically with languages from the [[IEC 61131-3]] standard that is common to many PLCs, RTUs and DCSs. In remote locations, it is common to use an RTU as a gateway for a PLC, where the PLC is performing all site control and the RTU is managing communications, time-stamping events and monitoring ancillary equipment. On sites with only a handful of I/O, the RTU may also be the site PLC and will perform both communications and control functions.