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PCI Express PCIe Expansion Slots
PCIe (PCI express) began replacing PCI, PCI-X, and AGP by 2005. Five slot sizes are available for PCIe: x1, x2, x4, x8, and x16. While x8 is an official standard, it is not very common and hard to find on both motherboards and cards. The two most common PCIe cards are the x1 and x16. PCIe x1 was designed to replace the old PCI cards for networking, modems, audio, input/output, and other needs by connecting to the southbridge. PCIe x16, on the other hand, focused on replacing AGP and PCI-X for high-speed graphics cards, and connects to the northbridge. Most motherboards support numerous types of PCIe, with several x1 and either one or two x16 card slots available. Notably, a smaller PCIe card can fit into larger PCIe slots, but not vice versa. For example, a PCIe x2 can fit into a PCIe x8, but a PCIe x8 cannot fit into a PCIe x2.
PCIe is considered the next generation I/O bus architecture. Rather than a shared bus, each PCIe slot links to a switch that prioritizes and routes data through a point-to-point dedicated connection and provides a serial full-duplex method of transmission. There are different versions of PCIe slots. The maximum number of lanes that can be assigned to the card inserted into the slot is based upon the card’s version. Basic PCIe provides one lane for transmission (x1), at a transfer rate of 250 MBps. It can also provide multiple transmission lanes (x2, x4, x8, x16, x32). Newer versions of PCIe can transfer data at an even higher rate per lane:
- PCIe Version 1: 250 MBps
- PCIe Version 2: 500 MBps
- PCIe Version 3: 1 Gbps
- PCIe Version 4: 2 Gbps
PCIe cards are designated both by size and bandwidth capability. For example, a card may be identified as “single-lane (x1) PCIe with throughput up to 2 Gbps”, which would indicate this card is an x1 size card, and a version 4 card, based on its data transfer rate. Note that “Gbps” refers to “gigabits per second,” the measurement of the rate of data transfer. This should not be confused with a “GB” or “gigabyte,” which refers to the storage capacity of a device. The amount of data lanes in a connection represents the amount of bandwidth between the card and the host: more lanes equals more bandwidth. PCIe x32 cards offer maximum bandwidth, but they are rarely used because of their size.
In addition to greatly increased speed, PCIe offers higher-quality service. PCIe is backwards compatible and allows legacy PCI technology to be run in the same system (i.e. you can have both PCIe and PCI buses in the same system). PCIe buses are most commonly used for video cards in modern computer systems, although nearly any other device can be designed for a PCIe slot.
Small form factor computers, such as laptops or micro-ATX systems, might include a mini-PCI slot. Mini-PCI devices are small cards with either 100 or 124 pins. A typical use for a mini-PCI slot is to add internal cards (such as wireless cards and cellular cards) to laptops.