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Tech Tip 96 - Taking the Mystery Out of PCI-Express

By Jack M. Germain

Sunday, Sept. 24, 2006

For the last few years, computer designers have been quietly transitioning computer users into a major technology upgrade. This new technology changes the way motherboards [define] handle data from other components plugged into the I/O data bus [Input-Output]. A bus is a connection point between the CPU [Central Processing Unit] and computing devices consumers want to add to their equipment.

The changeover was supposed to be seamless for consumers, and in most cases, it has been. But when consumers buy new computers and try to plug in some of their older “legacy” devices such as scanners, graphics cards or specialized audio boards, sometimes the older gear does not work properly. Likewise, when consumers buy replacement audio or graphics cards or other devices they discover that the circuit board does not fit the newer I/O (Input/Output) slot on the motherboard. When this happens, more likely than not, the culprit is the new PCI-Express [PCE-E] slot.

Often, there is nothing that can be done to make mismatched equipment work together. There is simply a physical difference in the connecting plug of the device and the interface socket that is supposed to accept it. Also, the device may fit the new PCI-Express slot but still will still not work properly. However, understanding the new PCI-Express technology may help you to find a work-around solution.


Out with the Old PCI

The staple for the I/O bus design has long been the Peripheral Component Interconnect or PCI slot. Motherboard manufacturers designed their products with enough PCI slots for basic components such as sound cards and video cards [printed circuit boards that control the audio and graphics functions] and usually included one or more extra PCI slots for future consumer expansion. Engineering advances brought modifications to the PCI bus through new bus structures known as PCI-X and the Accelerated Graphics Port or AGP.

Each of these advancements brought better performance through faster signal delivery between the connected device and the computer’s memory and processor chip circuitry. But computer processors and memory systems eventually became so fast that the throughput capacity of the PCI bus reached its design limitations. A big reason for performance slowdowns in even newer, faster computers was the bottleneck of data flow between these peripheral devices and the motherboard.

This slowdown in performance was especially a problem for users of games, music and video programs that required massive quantities of graphics data. According to Gordon Burke, vice president of marketing for NextIO, the PCI bus was limited to parallel architecture. This means that the bus was designed to move signals along parallel lines of wires that handled a maximum of 8-bits of data. Engineers could tweak the data flow speed by providing more wired paths, but this method could not overcome the physical limitations of the 8-bit bus.

Computer engineers devised the PCI-E or Express bus to speed up the data flow. Instead of using parallel architecture, Burke said the Express bus uses serial technology which links data lines in series. This enables data to travel along different pathways depending on whether it is a signal coming in from the peripheral device to the CPU or returning from the CPU to the peripheral device.

“This serial bus architecture lets data go much faster on single lines than it could on parallel lines that had to share directional flow,” said Burke. “This PCI-Express solution has become so ubiquitous in the computer industry that it is used all the way down to hand-held devices.”


PCI-Express: A New Architecture

Think of the PCI-Express bus as a data router. You are probably familiar with a router [define] as the box that sits between your cable or DSL modem and two or more computers. Instead of having to pay for several separate Internet connections, the router lets you share your one paid connection with multiple computers. The router allows each computer to access the Internet as if it were the only computer connected.

The PCI-E slots can speed up the flow of digital information between your peripheral devices and the CPU because each one has dedicated bandwidth [the amount of signal allowed] to the computer’s memory. The older PCI slots had to share bandwidth, but the new PCI-Express slots can link together to provide a more direct and faster connection. The PCI-Express standard can also work with existing software designed for the older PCI bus. However, since it uses a physical bus capable of handling a high-speed (2.5 Gb/s) data signal, the connecting slots themselves are not compatible.

To solve this potential legacy problem, computer designers provided for a transition from traditional PCI to the new PCI-Express standard by designing motherboards with a combination of PCI and PCI Express connectors. Consumers can plug smaller connectors from PCI devices into the larger PCI-E host connectors on the motherboard. However, the newer PCI-E devices do not fit into the smaller sized PCI connectors.


Size Matters!

Motherboard design today includes these standard PCI-Express slot sizes. The x1 slot is a general-purpose slot that can host new PCI-E single-channel devices. The x4, x8, and x16 slots provide 4-channel, 8-channel and 16-channel extensions for use with devices that plug into desktop and server computers.

To better understand the physical differences between the older PCI slot and the newer PCI-Express, look at the graphic below. You can see the three traditional long PCI slots on the left side of the motherboard. The PCI-E x4 slot is colored black and sits in the upper center portion of the motherboard. To its right are two smaller, white-colored PCI-E x1 slots. To the far right of the motherboard is the long, blue-colored PCI-E x16 slot. Notice that the x16 slot is slightly larger than the traditional PCI slot.



Prevent or Fix Compatibility Issues

Sometimes, because of the physical and signal bandwidth incompatibility between PCI and PCi-E, the only fix is to simply buy a newer component that is designed to work with the available PCI-E slots when no PCI slots exist. When you buy an add-on product – such as a better graphics card to enhance your game playing pleasure – make sure that it matches the slots on your motherboard.

One last possible cure to consider for PCI-E compatibility issues is to check the device manufacturer’s website. Chances are pretty good that other consumers have already reported the kind of problems you are having. The manufacturer might have solved the problem with a newer device driver you can download and install. Another solution could be specific directions from the device’s manufacturer about changing driver settings or changing switches on the motherboard.


In Conclusion

The PCI-Express standard is here to stay and it is gradually replacing earlier bus interfaces. For the foreseeable future, motherboard makers will provide a variety of PCI-E slots along with the legacy PCI slots. For now it will usually not matter what upgrade component you buy because your computer’s motherboard will probably have a matching slot for it. Read our Tech Tips 15, 16 & 17 for more information on expansion card formats.

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