Editing Process control network (section)

==Development of standards== 
Many early PCNs were serial based, using low level standards such as [[Electronic Industries Alliance|EIA]] [[RS-422]] and EIA [[RS-485]] with proprietary [[Communications protocol|protocols]] on top. One of the [[de facto]] [[standardization|standard]]s (which is now becoming an [[open standard]]) is [[Modbus]], originally from Modicon. Many PCNs used [[token passing]] based protocols because they are essentially deterministic. Both [[Allen Bradley]] and [[Eurotherm]] utilised such mechanisms. Many of the measurement and control unit manufacturers signed up to the Fieldbus consortium but rather than one standard emerging each manufacturer promoted their own 'standard' leading to a myriad, and confusing, range of physical and [[logical system]]s. Modern process control networks rely on Ethernet, TCP/IP, and Microsoft Windows technology.<ref>{{cite web|url=http://energy.gov/sites/prod/files/Good%20Practices%20Guide%20for%20Firewall%20Deployment.pdf|title=Good practice guide on firewall deployment for SCADA|date=15 February 2005|website=energy.gov|access-date=30 July 2023}}</ref>