Computer motherboards contain several different types of expansion slots.
Expansion slots in a computer can be used for adding various capabilities to the computer that are not built-in. There are several types of expansion slot, from different computing eras, that allow users to upgrade their machine with modems, video cards, sound cards, and other types of expansion cards. This allows customization of a computer for use with different applications.
ISA
ISA, or "Industry Standard Architecture," is a type of expansion bus that was originally introduced in 1981 in an 8-bit version. In 1984, a 16-bit version was introduced, and ISA then became widely used in personal computers. ISA supports clock speeds of 8 and 33 MHz for both 8- and 16-bit versions. ISA slots were used as the most common expansion bus during the 1980s and 1990s. They were eventually replaced by faster PCI and AGP expansion bus technologies. Most modern motherboards no longer include ISA slots, but you will see them occasionally on older machines.
PCI
PCI stands for "Peripheral Component Interconnect." Originally designed by Intel, it is used in both PCs and Macintosh computers. Most common expansion cards use a PCI connection. Older video cards use PCI, but modern video cards use AGP or PCIe busses for their faster speeds. PCI slots are 3.5 inches long and .5 inch high, run at 33 MHz, and have a maximum data transfer rate of 132 MB per second.
AGP
AGP is shorthand for "Accelerated Graphics Port." AGP is expansion bus technology specifically designed for video cards. Originally designed by Intel, AGP slots offer faster transfer rates than standard PCI slots, able to transfer a maximum of 528 MB per second, versus 132 MB per second for PCI. This extra speed allows applications and 3D games to use more sophisticated graphics and geometry. AGP cards also have the ability to store graphics in system memory as well as on-board video memory.
PCI Express
Another way to describe PCI Express is "PCIe." PCIe works on a different architecture than standard PCI, which is a parallel system. Utilizing a network of serial connections controlled by a hub on the motherboard, PCIe busses run at significantly higher speeds than older PCI and AGP technologies. PCIe also comes in extended variations such as x1, x4, x8, and x16. These implementations increase the bus bandwidth by the corresponding amount, but require longer PCI Express slots. PCIe is eventually expected to replace both PCI and AGP busses in computers. However, PCIe is backwards compatible with standard PCI.
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