There are two types of mechanical design for PLC systems. A single box, or 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. Second design type – a modular PLC – 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 – which all can be
customized for the particular application.[13] 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. Options are also available to
mount I/O points directly to the machine and utilize quick disconnecting cables to sensors and valves, saving time for wiring and replacing components.[citation
needed]