The diagram on this slide conceptualizes a typical programmable logic controller which can be segmented by I/O count and scan rates. They can also be classified as brick type versus modular PLCs. A small or brick PLC has a fixed number of connections for inputs and outputs. Modular PLCs (like the one shown here) have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customized to fit the application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.