[[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 twofour 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}}The third design type{{snd}} a ''Stackable or Rack-Mounted'' {{snd}} PLC, this design combines the features of both single box and modular PLCs. The CPU, power supply, I/O interfaces, and other components are separate modules that connect via cables and can be stacked on top of each other. This structure allows for flexible configuration and can be made compact in size. The forth design type a ''Distributed PLC'' PLC, This system uses multiple PLCs or I/O modules distributed across a network, communicating with each other and with a central controller.<ref>{{cite web |title=History, Basic Structure & Classification of Programmable Logic Controller (PLC) |date=24 August 2023 |url=https://www.plc-hub.com/history-basic-structure-classification-of-programmable-logistic-controller-plc/ |website=plc-hub.com |access-date=12 December 2024}}</ref>
