What is PCI Express?

Peripheral Component Interconnect (PCI) Express (PCIe) is a scalable Input/Output (I/O) serial bus technology that largely replaced earlier PCI slots on motherboards. It is a port that allows certain internal components to be installed into a computer. In 2004, PCI Express slots began appearing alongside standard slots, starting a gradual transition to the new technology. While some standard PCI slots can still be found on motherboards, many computer users prefer PCI express for graphics cards and other components.

PCIe slots are found on many motherboards, letting computer users install components into them. They allow the motherboard and other software in a computer to access and use devices connected to these slots. While PCIe has been used throughout the first decade of the 21st Century, new slots are likely to replace them at some point in the future.

PCI Express is a point-to-point connection, which means it does not share bandwidth but communicates directly with devices via a switch that directs data flow. This allows for "hot swapping" or "hot plugging," which means cards in PCIe slots can be changed without shutting down the computer, and they consume less power than previous PCI technology. One of the most promising features of PCIe is that it is scalable, which means greater bandwidth can be achieved through adding more "lanes."

PCI Express has several additional advantages, not only to the user but to manufacturers. It can be implemented as a unifying I/O structure for desktops, servers, and workstations, and it is cheaper than PCI standard to implement at the motherboard level. This keeps costs low for the consumer. It is also designed to be compatible with earlier Operating Systems and PCI device drivers.

The initial rollout of PCI Express provided three consumer options: x1, x2, and x16. These numbers represents the "lanes:" x1 has 1 lane; x2 has 2 lanes, and x16 has 16. Each lane is bi-directional and consists of 4 pins. Lanes in PCIe version 1.x had a lower delivery transfer rate, but PCIe 3.0 introduced a transfer rate of 500 megabytes per second (MBps) in each direction for a total of 1,000 MBps, or 1 gigabyte per second (GBps), per lane.

The 16-lane (x16) slot has replaced the Accelerated Graphics Port (AGP) on many motherboards and fits a PCIe graphics card. Boards that include the x1 and x2 slots usually have them for other components, such as sound or networking cards. As computer graphics demands increase, x32 and x64 slots may become available, and future versions of PCIe might improve upon lane data rates.

PCI Express should not be confused with PCI eXtended (PCI-X), used in the server market. PCI-X improved on standard PCI bus to deliver a maximum bandwidth of 1GBps. PCIe has been developed for the server market as well, initially with the x4, x8 and x12 formats reserved. This far exceeds PCI-X capability.

Intel first introduced PCI technology in the early 1990s to replace the Industry Standard Architecture (ISA) bus. Although robust enough to last over a decade, total available bandwidth of just 133 MBps, shared between slots, meant that high demand devices quickly overwhelmed computer resources. In 1997 this problem was partially alleviated by implementation of a separate AGP slot with dedicated bandwidth. However, as component manufacturers developed many high-demand devices for computers, a new architecture was required, which led to the introduction of PCI Express.