PCI-X vs. PCI-Express

Created: November 08, 2004

Erik Vlietinck

PCI-X 2.0 is the next generation of PCI. PCI Express architecture is an industry standard high-performance, general-purpose serial I/O interconnect designed for use in enterprise, desktop, mobile, communications and embedded platforms. It has many state-of-theart attributes which PCI-X hasn’t. Apple chose to implement PCI-X in its earlier Power Mac G5 machines. Which makes you wonder if this was the right decision.

By the looks of the market and products available, it seems Apple took the right decision to implement PCI-X and not PCI-Express. Although Apple’s approach could be described as conservative, which is not what users expect from the company, conservatism is what pays off when it comes to these two technologies.

PCI-Express products are scarce. nVidia is one of the few companies which have a product line-up that is PCI-Express compatible. nVidia says it is working hand-in-hand with Intel in the development of graphics solutions for the new graphics bus architecture.

nVidia products that are PCI-Express ready, are: GeForce 6800 (select versions), GeForce 6600, GeForce PCX, NVIDIA nForce4 SLI, NVIDIA nForce4 Ultra, NVIDIA nForce4, NVIDIA Quadro FX 4400, NVIDIA Quadro FX 4400G, NVIDIA Quadro FX 3400, NVIDIA Quadro FX 1400, NVIDIA Quadro FX 1300, NVIDIA Quadro FX 540, NVIDIA Quadro FX 330, and NVIDIA Quadro NVS 280 (select versions).

However, jumping on the PCI-Express bandwagon now seems a bit premature. Gartner Group reports that on Intel’s product front, negative news dominated the past three months. Not only did Intel confirm that its fastest desktop microprocessors were in short supply and that the 4-gigahertz variant would be delayed, Intel’s newest chipset for workstation and server markets had bugs in its PCI Express interface. This implies that few PCs have PCI-Express on-board.

Furthermore, PCI-X is not yet an end-of-life technology. The graphic below shows there still is some life in PCI-X beyond what is offered in Power Mac G5 and other PCI-X systems.

The rest of this article is based on publicly available papers and reports concerning PCI-X and PCI-Express, mainly to be found at PCI-SIG’s web site. For more in-depth coverage of these technologies, please refer to Accelerate Your Mac, and Ars Technica.

PCI-X speed

Brief overview of PCI-X

PCI-X builds upon previous generations of PCI including electricals, protocols, signals names, etc. It maintains backward compatibility with conventional PCI and is the next logical advance in the popular PC bus. There have been many generations of PCI, which all build upon each other, and PCI-X is no exception.

To increase the bus speed and reduce latency PCI-X 1.0 was developed, with a maximum clock speed of 133 MHz. PCI-X 1.0 also introduced improved protocols, such as the split-transaction protocol which allows more efficient use of bus bandwidth, resulting in throughput gains beyond the simple increases in clock speed and bus width.

Because of the demand for even higher throughput and to improve error correction, the PCI-X 2.0 specification was developed. It extends the bus frequency to 266 MHz and 533MHz and adds advanced features like ECC, while still maintaining backward compatibility to the first generation.

There are 4 speed grades in the PCI-X 2.0 specification: PCI-X 66, PCI-X 133, PCI-X 266, and PCI-X 533. The PCI-X 66 and PCI-X 133 speed grades were included in the PCI-X 1.0 specification. 100MHz PCI-X has been implemented in the market by using PCI-X 133 adapter cards. Both PCI-X 266 and PCI-X 533 are new to PCI-X 2.0; they are the 266MHz and 533MHz versions of the specification.

All four speed grades are included in the PCI-X 2.0 specification. PCI-X 2.0 is backward compatible with all generations of PCI. PCI-X 266 and PCI-X 533 devices are electrically compatible with 3.3V and 1.5V I/O buffers only. They are not compatible with 5V PCI. The latest version of the PCI local-bus specification (v3.0) obsoletes 5V-only add-in cards and 5V-only system slots. 

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