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PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards. PCIe has numerous improvements over the aforementioned bus standards, including higher maximum system bus throughput, lower I/O pin count and smaller physical footprint, better performance-scaling for bus devices, a more detailed error detection and reporting mechanism, and native hot plugging. More recent revisions of the PCIe standard support hardware I/O virtualization.

The PCIe electrical interface is also used in a variety of other standards, most notably ExpressCard, a laptop expansion card interface.

Format specifications are maintained and developed by the PCI-SIG (PCI Special Interest Group), a group of more than 900 companies that also maintain the Conventional PCI specifications. PCIe 3.0 is the latest standard for expansion cards that is available on mainstream personal computers.
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History

While in development, PCIe was initially referred to as HSI (for High Speed Interconnect), and underwent a name change to 3GIO (for 3rd Generation I/O) before finally settling on its PCI-SIG name PCI Express. It was first drawn up by a technical working group named the Arapaho Work Group (AWG) which, for initial drafts, consisted of an Intel only team of architects. Subsequently the AWG was expanded to include industry partners.

PCIe is a technology under constant development and improvement. The current PCI Express implementation is version 3.0.

PCI Express 1.0a

In 2003, PCI-SIG introduced PCIe 1.0a, with a data rate of 250 MB/s and a transfer rate of 2.5 GT/s.

PCI Express 1.1

In 2005, PCI-SIG introduced PCIe 1.1. This updated specification includes clarifications and several improvements, but is fully compatible with PCI Express 1.0a. No changes were made to the data rate.


PCI Express 2.0

PCI-SIG announced the availability of the PCI Express Base 2.0 specification on 15 January 2007.[10] The PCIe 2.0 standard doubles the per-lane throughput from the PCIe 1.0 standard's 250 MB/s to 500 MB/s. This means a 32-lane PCI connector (x32) can support throughput up to 16 GB/s aggregate. The PCIe 2.0 standard uses a base clock frequency of 2.5 GHz, while the first version operates at 1.25 GHz.

PCIe 2.0 motherboard slots are fully backward compatible with PCIe v1.x cards. PCIe 2.0 cards are also generally backward compatible with PCIe 1.x motherboards, using the available bandwidth of PCI Express 1.1. Overall, graphic cards or motherboards designed for v 2.0 will be able to work with the other being
v 1.1 or v 1.0.

The PCI-SIG also said that PCIe 2.0 features improvements to the point-to-point data transfer protocol and its software architecture.

Intel 's first PCIe 2.0 capable chipset was the X38 and boards began to ship from various vendors (Abit, Asus, Gigabyte) as of October 21, 2007.[12] AMD started supporting PCIe 2.0 with its AMD 700 chipset series and nVidia started with the MCP72.[13]All of Intel's prior chipsets, including the Intel P35 chipset, supported PCIe 1.1 or 1.0a.

PCI Express 2.1

PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned to be fully implemented in PCI Express 3.0. However, the speed is the same as PCI Express 2.0. Most motherboards sold currently come with PCI Express 2.0 connectors.

PCI Express 3.0

PCI Express 3.0 Base specification revision 3.0 was made available in November 2010, after multiple delays. In August 2007, PCI-SIG announced that PCI Express 3.0 would carry a bit rate of 8 gigatransfers per second, and that it would be backwards compatible with existing PCIe implementions. At that time, it was also announced that the final specification for PCI Express 3.0 would be delayed until 2011, although more recent sources stated that it may be available towards the end of 2010. New features for the PCIe 3.0 specification include a number of optimizations for enhanced signaling and data integrity, including transmitter and receiver equalization, PLL improvements, clock data recovery, and channel enhancements for currently supported topologies.

Following a six-month technical analysis of the feasibility of scaling the PCIe interconnect bandwidth, PCI-SIG's analysis found out that 8 gigatransfers per second can be manufactured in mainstream silicon process technology, and can be deployed with existing low-cost materials and infrastructure, while maintaining full compatibility (with negligible impact) to the PCIe protocol stack.

PCIe 2.0 delivers 5 GT/s, but employs an 8b/10b encoding scheme which results in a 20 percent overhead on the raw bit rate. PCIe 3.0 removes the requirement for 8b/10b encoding and instead uses a technique called "scrambling" in which "a known binary polynomial is applied to a data stream in a feedback topology. Because the scrambling polynomial is known, the data can be recovered by running it through a feedback topology using the inverse polynomial"[17] and also uses a 128b/130b encoding scheme, reducing the overhead to approximately 1.5%, as opposed to the 20% overhead of 8b/10b encoding used by PCIe 2.0. PCIe 3.0's 8 GT/s bit rate effectively delivers double PCIe 2.0 bandwidth. According to an official press release by PCI-SIG on 8 August 2007:

"The final PCIe 3.0 specifications, including form factor specification updates, may be available by late 2009, and could be seen in products starting in 2010 and beyond."

As of January 2010, the release of the final specifications had been delayed until Q2 2010. PCI-SIG expects the PCIe 3.0 specifications to undergo rigorous technical vetting and validation before being released to the industry. This process, which was followed in the development of prior generations of the PCIe Base and various form factorspecifications , includes the corroboration of the final electrical parameters with data derived from test silicon and other simulations conducted by multiple members of the PCI-SIG.
On May 31, 2010, it was announced that the 3.0 specification would be coming in 2010, but not until the second half of the year.[20] Then, on June 23, 2010, the PCISpecial Interest Group released a timetable showing the final 3.0 specification due in the fourth quarter of 2010.

Finally, on November 18, 2010, the PCI Special Interest Group officially publishes the finalized PCI Express 3.0 specification to its members to build devices based on this new version of PCI Express.

Current status

PCI Express has replaced AGP as the default interface for graphics cards on new systems. With a few exceptions, all graphics cards being released as of 2009 and 2010 from AMD (ATI) and NVIDIA use PCI Express. NVIDIA uses the high bandwidth data transfer of PCIe for its Scalable Link Interface (SLI) technology, which allows multiple graphics cards of the same chipset and model number to be run in tandem, allowing increased performance. ATI has also developed a multi-GPU system based on PCIe called CrossFire. AMD and NVIDIA have released motherboard chipsets which support up to four PCIe ×16 slots, allowing tri-GPU and quad-GPU card configurations.

Uptake for other forms of PC expansion has been much slower and conventional PCI remains dominant. PCI Express is commonly used for disk array controllers, onboard gigabit Ethernet, and Wi-Fi but except for graphics cards, add-in cards are still generally conventional PCI, particularly at the lower end of the market. Sound cards, TV/capture-cards, modems, serial port/USB/Firewire cards, network/WiFi cards and other cards with low-speed interfaces are still nearly all conventional PCI. For this reason most motherboards supporting PCI Express offer conventional PCI slots as well. As of 2010 many of these cards are starting to make their way over to x8, x4, or x1 PCIe slots which are present in motherboards. For instance, almost all new sound cards from the second half of 2010 are now PCIe.

ExpressCard has been introduced on several mid- to high-range laptops such as Apple's MacBook Pro line. Unlike desktops, however, laptops frequently only have one expansion slot. Replacing the PC card slot with ExpressCard slot means a loss in compatibility with PC-card devices.

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