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A '''distributed control system''' ('''DCS''') is a computerized [[control system]] for a process or plant usually with many [[control loop]]s, in which autonomous controllers are distributed throughout the system, but there is no central operator supervisory control. This is in contrast to systems that use centralized controllers; either discrete controllers located at a central control room or within a central computer. The DCS concept increases reliability and reduces installation costs by localizing control functions near the process plant, with remote monitoring and supervision.
Distributed control systems first emerged in large, high value, safety critical process industries, and were attractive because the DCS manufacturer would supply both the local control level and central supervisory equipment as an integrated package, thus reducing design integration risk. Today the functionality of [[SCADA|Supervisory control and data acquisition (SCADA)]] and DCS systems are very similar, but DCS tends to be used on large continuous process plants where high reliability and security is important, and the control room is not necessarily geographically remote. Many machine control systems exhibit similar properties as plant and process control systems do.<ref>{{Cite book |last=Eloranta |first=Veli-Pekka |title=Designing distributed control systems: a pattern language approach |last2=Koskinen |first2=Johannes |last3=Leppänen |first3=Marko |last4=Reijonen |first4=Ville |date=2014 |publisher=Wiley |isbn=978-1-118-69415-2 |series=Wiley series in software design patterns |___location=Chichester}}</ref>
==Structure==
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* [[Chemical plant]]s
* [[Petrochemical]]
* [[Pulp and paper mill]]s (see also: [[Quality Control System QCS for paper, board and tissue machines|quality control system QCS]])
* Boiler controls and [[power plant]] systems
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The drive toward openness in the 1980s gained momentum through the 1990s with the increased adoption of [[commercial off-the-shelf]] (COTS) components and IT standards. Probably the biggest transition undertaken during this time was the move from the UNIX operating system to the Windows environment. While the realm of the real time operating system ([[RTOS]]) for control applications remains dominated by real time commercial variants of UNIX or proprietary operating systems, everything above real-time control has made the transition to Windows.
The introduction of Microsoft at the desktop and server layers resulted in the development of technologies such as [[OLE for process control|OLE for process control (OPC)]], which is now a de facto industry connectivity standard. Internet technology also began to make its mark in automation and the world, with most DCS HMI supporting Internet connectivity. The 1990s were also known for the "Fieldbus Wars", where rival organizations competed to define what would become the IEC [[fieldbus]] standard for digital communication with field instrumentation instead of 4–20 milliamp analog communications. The first fieldbus installations occurred in the 1990s. Towards the end of the decade, the technology began to develop significant momentum, with the market consolidated around Ethernet I/P, Foundation Fieldbus and Profibus PA for process automation applications. Some suppliers built new systems from the ground up to maximize functionality with fieldbus, such as [[Rockwell Automation|Rockwell]] PlantPAx System, [[Honeywell]] with [[Experion]] & Plantscape [[SCADA]] systems, [[ABB]] with System 800xA,<ref>{{cite web|url=
The impact of COTS, however, was most pronounced at the hardware layer. For years, the primary business of DCS suppliers had been the supply of large amounts of hardware, particularly I/O and controllers. The initial proliferation of DCSs required the installation of prodigious amounts of this hardware, most of it manufactured from the bottom up by DCS suppliers. Standard computer components from manufacturers such as Intel and Motorola, however, made it cost prohibitive for DCS suppliers to continue making their own components, workstations, and networking hardware.
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