Selecting the Right PC for Your PCI Express Hardware

Selecting a PC today requires more than just choosing the largest hard drive and fastest processor. Some of the common requirements that engineers consider when selecting a PC include processor speed, available RAM, hard drive size, and number of peripheral device slots available. Another important consideration is the peripheral bus architecture. PCI, AGP, PCI-X, and ISA are common bus architectures found in PCs. This white paper examines the next generation of the PCI peripheral architecture, PCI Express. Learn about the technology behind PCI Express and what to consider when purchasing a PC with a PCI Express peripheral architecture.

Overview of PCI Express

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 generation of PCs.


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 vision systems, modular Ethernet storage, and high-channel-count digital I/O.

For vision applications, bandwidth has always been a limiting factor. Even relatively high-speed peripheral buses such as IEEE 1394 still rely on the PCI bus to deliver images to the processor. With new PCI Express vision products, such as the National Instruments PCIe-1429 Camera Link board, 680 MB/s per direction per device of bandwidth is available across two Camera Link cables. Also, PCI Express is a critical technology enabling Gigabit Ethernet. Without PCI Express, the bandwidth of Gigabit Ethernet would easily overwhelm the PCI bus.

Which PCs Support PCI Express?

Table 2 shows systems that support PCI Express from major PC manufacturers as of June 2005. The product portfolios from these companies are subject to change. This partial list of currently available PCs illustrates a few trends and serves as an overview of the types of desktop, server, and workstation PCs on the market today.


Trends for these new PCs include:

1. PC vendors are adopting PCI Express.
2. PCI and PCI Express exist side-by-side in these new systems.
3. PCI Express implementation varies among desktops, workstations, and servers.

The adoption of PCI Express is being driven by consumer graphics and higher-bandwidth slots for server-class systems. However, it is also important to highlight that PCI and PCI Express remain side-by-side. This is because bridging from PCI Express to PCI is easy with commercial bridges, and the large installed based of PCI creates a big incentive for PC vendors to continue to offer PCI. In fact, on public roadmaps at www.intel.com/products/roadmap, Intel shows PCI available on chipsets for the next six years.

The link widths of PCI Express slots vary among desktops, workstations, and servers because these systems serve different markets. Desktop systems 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 not available in the server class systems because high-end graphics is not an important feature for servers, although more bandwidth for peripherals such as additional Gigabit Ethernet ports is important.

How to Select the Appropriate PC with PCI Express

When selecting the proper PC for your PCI Express card, there are four factors to keep in mind.

1. A PCI Express slot that is the size of your PCI Express card is recommended. If you have a x4 PCI Express card, purchase a computer with a x4 slot.
2. If you have a normal-sized PCI Express card, check the specifications on “low-profile” PCI Express systems to make sure that your PCI Express card will fit.
3. In general, avoid the x16 PCI Express slot for non-graphics usage. In many cases, these slots are designed for use with graphics cards.
4. A limited number of PCs have a x8 PCI Express connector that only includes four lanes. If you purchase one of these PCs, make sure you are planning to use a x4 or x1 PCI Express card. The Dell Precision Workstation 470, the Dell Precision Workstation 670, the HP xw6200, and the HP xw8200 all have a x4 PCI Express slot with a x8 connector.

Up-Plugging and Down-Plugging
As mentioned above, plugging a x4 PCI Express board into an x8 slot is acceptable. This 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 only requires slots to negotiate 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 an x8 PCI Express slot, your board might operate at x1 bandwidth even though it is capable of 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.)

PCI Express was designed to incorporate the AGP graphics card back into the common peripheral bus that computers use. For this reason, the x16 slot with 4 GB/s per direction of available bandwidth is generally reserved for graphics cards and, in many cases, is not available for peripheral device use. In addition, the first peripheral devices on the market were designed as either a x1 (250 MB/s per direction) or as a x4 (1 GB/s per direction). The ability to scale bandwidth with each device in the system means that the majority of peripheral devices, in the short term, are either a x1 or x4 device.

It is important to note that PCI and PCI Express remain side-by-side, as all of the PCs listed in Table 2 contain both PCI/PCI-X and PCI Express slots. A few of the reasons that the PCI bus continues to coexist with PCI Express include the following:

1. PCI boards make up a majority of the market. It is estimated that more than 1 billion PCI boards were sold in 2004.
2. PCI Express and PCI offer the same software model, so bandwidth is the driving differentiator between them, and there is not a new software architecture that addresses plug-and-play technology to drive a faster transition like the ISA-to-PCI transition.
3. There is limited need for PCI Express bandwidth.

   Glossary
   

advanced graphics port (AGP) – A higher-speed version of the PCI bus using a different connector. Developed to accommodate the bandwidth needs of dedicated plug-in graphics cards in desktop PCs.

bandwidth – The amount of data that can be transferred through a digital connection in a given time period.

Camera Link – A technology used to connect high-performance machine vision and scientific cameras to image acquisition devices. Camera Link replaces traditional parallel digital cables that are bulky, fragile, and expensive with an inexpensive, standardized cable and connector.

Ethernet – A frame-based computer networking technology for local area networks (LANs).

peripheral component interconnect (PCI) – A high-speed parallel bus originally designed by Intel to connect I/O peripherals to a CPU.

PCI Express – An evolutionary version of PCI that maintains the PCI software usage model and replaces the physical bus with a high-speed (2.5 Gb/s) serial bus serving multiple lanes.

peripheral component interconnect extended (PCI-X) – A variation of the PCI bus architecture with increased clock frequency and moderate increase in bandwidth.

random-access memory (RAM) – Typically used for primary storage (main memory) in computers to hold actively-used and actively-changing information.
Related Links:
National Instruments PCI Express Resources
PCI Express – An Overview of the PCI Express Standard
Intel PCI Express Resources
PCI-SIG Home
ExpressCard Home

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