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Many early PLC programming applications were not capable of graphical representation of the logic, and so it was instead represented as a series of logic expressions in some kind of Boolean format, similar to [[Boolean algebra]]. As programming terminals evolved, because ladder logic was a familiar format used for electro-mechanical control panels, it became more commonly used. Newer formats, such as state logic,<ref>{{cite web |url=https://control.com/technical-articles/state-machine-programming-in-ladder-logic/ |title=State Machine Programming in Ladder Logic |access-date=2024-08-18}}</ref> [[function block diagram]]s, and [[structured text]] exist. Ladder logic remains popular because PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the person writing the logic to see any issues with the timing of the logic sequence more easily than would be possible in other formats.<ref>{{cite web |title=Wrapping Your Head around Ladder Logic |date=27 August 2018 |url=https://www.dosupply.com/tech/2018/08/27/wrapping-your-head-around-ladder-logic/ |website=DoSupply.com |access-date=19 October 2020}}</ref>
Up to the mid-1990s, PLCs were programmed using proprietary programming panels or special-purpose programming [[Computer terminal|terminals]], which often had dedicated function keys representing the various logical elements of PLC programs.<ref name=":9" /> Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain [[ASCII art|ASCII]] character representations of contacts, coils, and wires were common. Programs were stored on [[cassette tape cartridge]]s. Facilities for printing and documentation were minimal due to a lack of memory capacity. The oldest PLCs used [[magnetic-core memory]].<ref>{{cite web |url=https://www.reddit.com/r/PLC/comments/17p2wzj/the_inside_of_the_core_memory_module_of_a_plc2_if/ |title=The inside of the core memory module of a PLC2, if anyone's interested. |access-date=2024-07-25}}</ref
==Architecture==
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* 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
* A communications interface to receive and transmit data on communication networks from
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" /><!--[[User:Kvng/RTH]]-->
===Mechanical design===
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