Editing Programmable logic controller (section)

===Mechanical design===
[[File:Siemens sps logo 8 12-24 RCE-03.jpg|thumb|Compact PLC with 8 inputs and 4 outputs]]
[[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 two 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}}