PCI X Graphics Card Interface Overview

The PCI-X Interface

Peripheral Component Interconnect eXtended (PCI-X) was released in 1998 as an adaptation and improvement of the PCI standard architecture.

PCI-X was designed as an overall speed boost to the original PCI protocols. While PCI-X maintained a 32 bit bus transfer rate, the bandwidth on PCI-X is nearly doubled over that of PCI and clock speed are increased from the 66 MHz maximum of late-generation PCI to 133 MHz in the initial run of PCI-X.

PCI-X was a significant increase at the time, and was adopted as an industry standard architecture shortly before the release of PCI-Express in 2004. With the emergence of PCI-Express, all legacy versions of PCI technology became obsolete, and PCI-X landed on the scrap heap. PCI-Express is now the architecture used by nearly all IBM compatible computers presently on the market.

Interface History

In the late 90's the computer industry was facing a quandary. There were many new data-hungry technologies that simply couldn't be adequately served by the aging PCI architecture:

The computer industry needed more speed at the motherboard to provide the computing power necessary to feed this new technology.

PCI-X was a joint innovation of IBM, Hewlitt Packard, and Compaq. Putting their heads together to address the data transfer shortcomings of PCI in a world that was rapidly becoming hungry for faster processing, these three companies effectively re-invented the PCI bus connection and provided room for significant increases of clock speed and bandwidth.

Advantages and Improvements

In standard PCI architecture, data that cannot be immediately processed by the CPU is kept in a waiting state until the CPU has time to address it. Since PCI lacks a split response mechanism to recall send data being kept waiting and resend it at a later time, which generates a bottleneck, or a data backup, at the CPU level.

However, in the PCI-X architecture, this split response mechanism is present, allowing data that is being held waiting to be dropped and resent when the CPU is not as busy.

This split response is only generated when the CPU sends a signal that it is ready to receive the data and provide instructions in return, meaning that the PCI-X split response mechanism all but eliminates the bottleneck that plagued PCI architecture since its initial release.

Overall system and speed performance increases of PCI-X over PCI are listed below:

Additionally, PCI-X is backwards compatible with most PCI cards of version 2.0 or later, provided the extended portion of the PCI-X card is able to fit unobstructed into the computer case. It should be noted that installing a legacy PCI card into a PCI-X motherboard will reduce the speed of all components presently installed in the system to the speed of the legacy card.

Version 2

PCI-X version 2.0 was released on to the market in 2003, a year before the emergence of PCI-Express. PCI-X2 increased maximum clock speeds to 533MHz, and increased maximum bandwidth to just over 4 gigs per second.

PCI-X2 also included several upgrades that increased the overall stability of the computer system by streamlining the internal split response mechanism to better handle resent data dropped or ignored by the CPU.

PCI-X vs PCI-Express

PCI-X is often confused with PCI Express, due not only to their phonetic similarities but to their relatively close release dates. However, these two architectures couldn't be more different. Notable diversities include:

PCI-X Video Card Interface Summary

While PCI-X was a great leap forward over standard PCI when it was released, the quick succession of PCI-Express has rendered the entire architecture obsolete. If PCI-X had been introduced sooner, or if PCI-Express had come around later, PCI-X technology might have made a more lasting impression on the computer industry.

PCI-Express is backwards compatible with most previous versions of standard PCI, meaning users with PCI-X cards could use them in PCI-Express systems, but the massive loss in speed and reliability are hardly worth it.

As it stands, PCI-X is essentially a dead technology, and it's simply not worth the hassle of keeping old PCI-X devices around.