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What is a PCIe Bus?

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  • Written By: Mike Howells
  • Edited By: Michelle Arevalo
  • Last Modified Date: 27 August 2016
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A peripheral component interconnect express bus, also known as a PCIe bus, is a computer part that allows a PCIe peripheral to plug into, and communicate with, a motherboard. PCIe itself is an updated version of older peripheral component interconnect (PCI) technology, which, in principle, allows data to flow between a peripheral and a motherboard. A PCIe bus can handle a much faster transfer of data than older PCI buses, which typically translates to better graphics or network connections.

The general idea behind PCI is to allow add-ons to be installed onto computers. A PCI or PCIe bus can be thought of as an expansion slot, into which modules can be plugged to directly enhance a particular capability of a computer. Common examples of PCIe peripherals are video graphics cards, sound cards, and network cards.

With the addition of a PCIe graphics card, for instance, the processing required to render graphics can be offloaded from a computer's central processing unit (CPU), and handled instead by the graphics card's own processor. This leaves the CPU with more capacity to handle other processing tasks. In most cases, purpose-built PCIe peripherals have their own random access memory (RAM) included as well, and do a much better job at their specific task than a generic CPU.

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A PCIe bus itself differs from a PCI bus in that data is sent in serial across dynamically available channels, as opposed to parallel communication across only a finite number of channels. This means that, not only can more than two devices communicate at the same time, but unused channels can be grouped together to provide extra bandwidth if necessary. In the same way, a PCIe bus can respond to decreased demand by turning off unnecessary channels and saving power.

Physically, PCIe connectors are a different size than both PCI connectors and accelerated graphics port (AGP) connectors, which are a third type of common peripheral slot. None are inter-compatible, and trying to force one type of card into the wrong type of connector risks damage to both. In addition there are different sizes of PCIe slots that can handle correspondingly different numbers of channels. These different sizes are represented by a multiplier in the name, and bigger numbers mean more channels, and longer physical length. Speeds range from 1x to 16x, but it is possible for a 16x-sized slot, for instance, to handle only 1x data transfer feeds. This is common in cheaper motherboards to allow greater physical compatibility without the extra cost associated with actually having to handle higher PCIe bus speeds. In such an example, the 16x device will work but only run at 1x speed.

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