I/O buses are used to connect the central processing unit and the main memory with secondary memories, peripheral devices and plug-in cards. All data that is read in by the input devices and output by the output devices runs via the I/O bus.
The I/O bus is an extension of the system bus that terminates on the motherboard in a controller chip and forms a connection to the I/O bus. It has a lower clock frequency than the system bus. The best known I/O buses are ISAbus, EISA bus, PCI bus, AGP bus and the USB interface as well as the HyperTransport on printed circuit boards.