Every peripheral device (keyboard, modem, display, etc.) requires a control circuit that interfaces with the computer's processor (CPU). These circuits are either built into the chips on the motherboard or are contained on plug-in cards. See chipset.
On the Motherboard
A fully equipped PC motherboard has built-in controller circuits for all the basic peripheral devices. It has sockets for the internal drives (floppy, CD/DVD, hard disk) and ports for external devices: keyboard, mouse, printer, joy stick, speakers, microphone and USB. The motherboard may also provide control for a display, modem, network and FireWire.
Via Plug-In Cards
Additional input/output can be added by plugging in controller cards into the motherboard's PCI or AGP bus. For example, a modem, network adapter or FireWire card can be added. Higher-performance cards can also replace built-in functions. Quite often, motherboards have a built-in controller for the monitor, but a higher-speed display adapter can be plugged in, and the built-in circuit can be disabled in the BIOS. See PCI and BIOS.
The Back of the Cabinet
A PC typically has all the ports seen in the following image. Serial ports are used for modems, digitizer tablets and other devices, while the parallel port is used for printers. Serial and parallel ports have fallen into legacy status and may not be on every PC. Most every peripheral that used to hook up via serial or parallel is now available as a USB device.
This is a fully equipped PC with ample input/output. Extra USB ports are often present on the front of the case, as well as FireWire and sound ports. In this PC, the display adapter (horizontal) contains outputs to a TV or VCR and is plugged into the AGP bus. The vertical bank of ports is mounted fast on the motherboard.
A plug-in card is required when the circuitry is not built into the motherboard. This diagram shows the typical devices used. All peripherals require driver software, which enables the operating system to work with each device.
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