The Motherboard and System Unit

The system unit is the box in which the main components of a computer system are housed. For a desktop computer, this is typically the box that contains everything except the keyboard, mouse, screen, printer and other peripherals. Some desktop computers and all notebook, laptop and mobile devices do not have a distinct system unit but instead house all the system unit components in a single case along with the screen. The typical system unit houses the following items:

• motherboard
• hard drives
• optical drives
• power supply
• expansion cards
• various port connectors

Motherboard

A motherboard is a printed circuit board and is also known as the mainboard or logic board. It contains places to attach the various components: CPU, memory, hard drive controller, graphics card, sound card, keyboard, and mouse. It may also provide connections for USB, firewire, serial, parallel and other ports.

Power Supply

The power supply converts AC power to low voltage DC power and regulates the various voltages needed by the various components of the computer. In desktop computers the power supply is contained in a box approximately 6"x6"x3", which also contains a fan to keep the power supply cool. In notebook computers the power is supplied by the battery. There is a converter inside the notebook to provide the correct voltages for each of the components. The battery is charged by an external power supply sometimes referred to as a brick, because it contains a brick shaped box with one wire going to the outlet and the other plugging into the computer.

There is also an external device called an uninterruptable power supply, or UPS, that your computer plugs into. It contains a battery that is charged from the power cord into the main outlet. It has several outlets on it that supply power to various components. It is similar to a power strip/surge protector except that the battery will allow your computer to continue running for a brief time when the household power goes off. This will allow you to shut down your computer in a timely manner if the main power is lost. A home UPS will typically have enough power to run a desktop computer and monitor for at least 10 minutes after power is lost.

Expansion Cards

Expansion cards can be plugged into the motherboard to provide extra features that your system does not have. Desktop computers normally have from two to six expansion slots depending on the size of the system unit. Most modern motherboards have many capabilities built into them so the need for expansion cards is not as great as it used to be. Probably the most common use of expansion cards is for high-end graphics. The typical graphics capability that is included on modern motherboards is quite adequate for most applications but is perhaps not good enough for three-dimensional animation used in advanced computer games. Most motherboards also do not support RAID or external SATA without the use of expansion cards. TV tuners are also typically not included in most computers and may need expansion cards.

Ports

In computer hardware a port refers to an interface between the computer and other computers or peripheral devices. Common ports include:

• serial port - a connector for connecting a device to a computer that communicates one bit at a time using serial communication. Most modern computers no longer have serial ports because other technology has superseded it. Common serial devices include external modems, pointing devices (mouse), keyboards, and printers.
• parallel port - a connection for connecting a device to a computer that communicates one byte at a time using parallel communication. Most modern computers no longer have parallel ports. The most common parallel device is the printer. Today however most printers connect via a USB port or through a network.
• PS/2 ports - these ports are used to connect a keyboard or mouse to a PC compatible computer.
• USB port - universal serial bus is a serial bus standard for interfacing devices to a host computer. USB allows hot swapping, the ability to connect and disconnect devices without rebooting the computer. Virtually all types of devices can be connected to computers via USB, including the keyboard, mouse, printer, hard drive, flash drive, digital camera, optical drives, etc. Most computers have several USB ports. For example, my desktop computer has five USB ports and my notebook has three.
• Firewire port - aka i.LINK port or IEEE1394 port is also a high speed serial bus designed for real-time data transfer. It is commonly used for data storage devices and virtually all digital camcorders have firewire ports for transferring digital video from the camera to a hard drive in real time.
• VGA port - video graphics array is a standard analog video port for connecting to an analog video display
• DVI port - digital visual interface is a port for connecting a digital monitor to a digital video interface. DVI typically gives a better image than VGA.
• SCSI port - small computer systems interface is a port for connecting storage devices to computers. SCSI is primarily used to connect hard drives to server systems but can also be used for tape drives, optical drives and scanners.

Further Info

howSTUFFworks How PC Power Supplies Work
Wikipedia Computer Power Supply
Wikipedia Expansion Cards
Wikipedia System Unit
Wikipedia Port (hardware)
Wikipedia Universal Serial Bus
Wikipedia IEEE 1394 Interface (firewire)
Wikipedia PC Motherboard