A high-speed peripheral interconnect from Intel introduced in 2002. Note that although sometimes abbreviated "PCX," PCI Express is not the same as "PCI-X" (see PCI-SIG and PCI-X for comparison). As a result of the confusion, "PCI-E" or "PCIe" is the accepted abbreviation.|
Initially used for high-speed display adapters, and intending to eventually replace the PCI and AGP buses entirely, PCI Express was designed to match the higher speeds of today's CPUs. It can accommodate Gigabit and 10 Gigabit Ethernet and even support chip-to-chip transfers.
Rather than the shared, parallel bus structure of PCI, PCI Express provides a switched architecture using serial communications channels, each made up of two differential wire pairs that provide 2.5 Gbits/sec in both directions. Up to 32 channels ("lanes") may be combined in x2, x4, x8, x16 and x32 configurations, creating a parallel interface of independently controlled channels. The bandwidth of the switch backplane determines the total capacity of a PCI Express implementation.
Internal in 2002, External in 2007
Similar in architecture to InfiniBand, PCI Express was initially designed for internal connections, whereas InfiniBand provides a true fabric architecture for external networks. However, enhancements that extended PCI Express outside of the box for several meters were developed in 2007. External PCI Express allows, for example, a laptop computer to hook up to an external display adapter and monitor, providing greater performance than the laptop's internal display adapter.
Originally called "Third Generation I/O" (3GIO), PCI Express is software compatible with PCI, but not plug compatible. It also uses slots of different lengths because of the combined lanes (see below). For a comparison of all PCI technologies, see PCI-SIG. See ExpressCard and PCI-X.
PCI Express slots are both shorter and longer than PCI and AGP. The x1 slot is a single-lane implementation, while the x16 slot combines 16 lanes for peripherals such as the display adapter.
This display adapter from NVIDIA was one of the first x16 PCI Express cards on the market. PCI Express, AGP and PCI cards all have different edge connectors to ensure that they fit into the appropriate slots. (Image courtesy of NVIDIA Corporation.)