PCI

PCI (Peripheral Component Interconnect) is a type of computer bus for attaching or inserting peripheral devices into a computer. The PCI standard was first proposed by Intel in 1990 and was widely implemented in computers by 1995. Today, the specifications for PCI and its variants are maintained by the PCI-SIG (PCI Special Interest Group), a consortium of over 900 companies.

PCI is a general-purpose connection standard designed to support multiple devices of various kinds, including graphics hardware, audio hardware, network hardware, and so on. Revisions of the PCI standard have added new features and performance improvements, including different bus speeds and bus widths.

PCI-X

PCI-X (Peripheral Component Interconnect eXtended) is a type of computer bus standard designed to supersede PCI. It is essentially a faster version of PCI, running at twice the speed, and is otherwise similar in physical implementation and basic design.

And thus PCI-X is generally backward compatible with PCI, meaning that, a PCI-X card can be installed in a PCI slot provided it has the correct voltage keying for the slot and (if inserting in a 32 bit slot) nothing obstructs the overhanging part of the edge connector. PCI and PCI-X cards can be intermixed on a PCI-X bus, but the speed will be limited to the speed of the slowest card (for this reason and the voltage compatibility issue most systems with PCI-X will have a normal PCI bus as well) . PCI-X improves the fault tolerance of PCI allowing, for example, faulty cards to be reinitialized or taken offline.

PCI-Express / PCI-e

PCIe (PCI Express) is a recently introduced standard for connecting devices to computers. It's software-compatible with PCI but has higher potential bandwidth and greater flexibility than PCI or AGP. The PCIe specification is also maintained by the PCI-SIG. For wider compatibility, and because the two standards share a common foundation, most new computers support PCIe and PCI.

A connection between a PCIe device and the system is known as a "link" and this link is built around a dedicated, bi-directional, serial (1-bit), point-to-point connection known as a "lane". A lane is capable of simultaneously transferring 250 MB/s of data in each direction. A link can use more than one lane at a time but all links compliant with the PCIe specification must minimally support single-lane connections, referred to as "x1" (pronounced "by-one") links.

For higher potential bandwidth, PCIe devices and systems can optionally support links using multiple simultaneous lanes – for example, a "x16" link uses 16 lanes. To support extra lanes, a PCIe card and slot must be designed to accommodate the extra electrical lines required (2 lines per lane). Card and slot types exist for x1, x4, x8, and x16 links.

Currently, the only devices that use a x16 link are graphics cards. Other devices typically don't require the high potential bandwidths provided by such a connection.