Selecting a PC today requires more than just choosing the largest hard drive and fastest processor. Some of the common requirements that you need to consider when selecting a PC include processor speed, available RAM, hard drive size, and the number of peripheral device slots available. Another important consideration is the peripheral bus architecture. As the PC has evolved, it has seen a variety of bus architectures such as PCI, AGP, PCI-X, and ISA. Since the introduction of PCI Express, selecting the right PC for your PCI Express device can quickly become overwhelming with varying connector sizes and specifications. This paper examines the technology behind PCI Express and the considerations you need to make when purchasing a PC with a PCI Express peripheral architecture.
PCI Express is an evolution of the most popular computer peripheral bus in history, PCI. The industry consortium chartered to preserve and develop the PCI specification, the PCI Special Interest Group (PCI-SIG), developed PCI Express to address the bandwidth requirements of peripheral devices such as Gigabit Ethernet, modular storage devices, and high-end graphics while maintaining complete software compatibility with existing OSs and PCI-based applications. PCI Express is intended to replace the accelerated graphics port (AGP) that was developed to address PCI bandwidth limitations. PCI Express delivers 30 times the bandwidth of PCI and brings graphics back to the peripheral device bus, while increasing video bandwidth for the next generations of PCs.
| Bus | Bandwitdth | Increase Compared to PCI |
| PCI | 132 MB/s | - |
| X1 PCI Express | 250MB/s | 1.8x |
| X4 PCI Express | 7.5X | 7.5x |
| 16X PCI Express | 30.3X | 30.3x |
Table 1 shows the scalable bandwidth that PCI Express delivers for different lane widths compared to PCI. For example, a x1 (“by one”) PCI Express slot delivers 250 MB/s in each direction while a x16 lane width supports 4 GB/s. A few applications that benefit from the increased bandwidth, economies of scale of PC technology, and software compatibility of PCI Express include high-speed data acquisition, vision systems, modular Ethernet storage, and high-channel-count digital I/O.
In January 2007, PCI-SIG announced the availability of the PCI Express 2.0 specification. It doubled the bus standard’s bandwidth from 250 to 500 MB/s and included improvements to the point-to-point data transfer protocol and its software architecture. The standard is still under development, with the 7.0 standard expected in 2025.
| PCI Express Architecture | Bandwidth Per Lane Direction |
| 1.x | 250 MB/s |
| 2.0 | 500 MB/s |
| 3.0 | ~1 GB/s |
| 4.0 | ~2 GB/s |
| 5.0 | ~4 GB/s |
| 6.0 | ~7.5 GB/s |
Table 2. PCI Express Specification Bandwidth per Lane.
Today, most PCs are shipped with at least one PCI Express slot. Unfortunately, some PC vendors make it difficult to determine if the computer you are considering has PCI Express slots. A specifications page or technical overview page, which is often not linked at the top level, generally lists available expansion slots. Oftentimes, a PC is shipped with a combination of both PCI and PCI Express expansion slots.
The link widths of PCI Express slots vary among desktops, workstations, and servers. Desktop systems often offer a x16 slot intended for graphics cards and a x1 slot for PCI Express peripherals. Workstation class systems offer a x16 slot for graphics and a x4 slot for higher-end PCI Express peripheral devices, while the server class systems offer both a x4 and x8 slot for high-end peripherals used in server markets. The x16 graphics slot is typically not available in the server-class systems because high-end graphics is not an important feature for servers, while more bandwidth for peripherals such as additional gigabit Ethernet ports is important.
When selecting the proper PC for your PCI Express board, keep the following five factors in mind:
A PCI Express slot that is the size of your PCI Express board is recommended. If you have a x4 PCI Express board, purchase a computer with a x4 slot.
If you have a normal-sized PCI Express board, check the specifications on “low profile” PCI Express systems to make sure that your PCI Express board fits. For more information, visit What Is a Low-Profile PCI Card?
In general, avoid the x16 PCI Express slot for nongraphics usage. In many cases, these slots are designed for use with graphics cards.
A limited number of PCs have a x8 PCI Express connector that includes only four lanes. If you purchase one of these PCs, make sure you are planning to use a x4 or x1 PCI Express board.
PCI Express 2.0 motherboard slots are backward-compatible with PCI Express 1.x. Also, PCI Express 2.0 boards are compatible with PCI Express 1.1 motherboards and run using all available bandwidth of PCI Express 1.1.
Up-Plugging and Down-Plugging
As mentioned above, plugging a x4 PCI Express board into an x8 slot is acceptable. This setup is called up-plugging. The opposite—plugging a x8 PCI Express board into an x4 slot—is called down-plugging and is not physically supported. Table 3 shows the different PCI Express board/connector configurations.
In all cases, up-plugging is physically possible; however, the PCI Express specification requires slots to negotiate only to a x1 link width. While it is possible for a motherboard manufacturer to incorporate up-plugging without bandwidth degradation, it is not required. For example, if you plug an x4 PCI Express board into a x8 PCI Express slot, your board might operate at a x1 bandwidth even though it is capable of a x4 bandwidth. This behavior depends on the motherboard manufacturer. If you are planning on up-plugging, contact the motherboard vendor to determine how its hardware handles up-plugging. The size of the PCI Express connector physically prevents down-plugging. A x8 PCI Express board is too large to fit into a x4 connector. (Note: The CompactPCI Express specification supports down-plugging.)
| Slot | |||||
| x1 | x4 | x8 | x16 | ||
| Card | x1 | Required | Required* | Required* | Required* |
| x4 | No | Required | Allowed* | Allowed* | |
| x8 | No | No | Allowed | Allowed* | |
| x16 | No | No | No | Allowed | |
* Up-Plugging
Table 3. Slot/Board Compatibility (Up-Plugging and Down-Plugging).
PCI Express has provided PCs with a higher-bandwidth and scalable peripheral bus. This scalable architecture has added new configuration options when purchasing a PC. While later specifications of PCI Express are backward-compatible, you still need to double-check the connector size and compatibility of your PCI Express before and PC before purchasing.