PCI Express

The future of PC's and PC components are always changing. What we are looking at now is a replacement for the PCI and the AGP bus in one go. Remember the old ISA bus? Chances are you do but you don't have a single component or a slot on your motherboard. The components required a better bus than ISA and so PCI was brought in. Then when PCI slots could not provide the performance required for top of the range graphics cards, the AGP bus was created. It has been this AGP bus which has been upgraded time after time to keep up with the demand from graphics cards of today's technology. Once again this technology jumps forward. And now we have PCI express.

PCI express doesn't really have any resemblance to the original PCI bus. Firstly the Original PCI bus was a Parallel data system and the new PCI express is a serial system. Although a serial system sends data behind each other and not all together, in can be clocked at a much higher speed. The PCI express subsystem consists of several PCI "lanes" 1 PCI lane was have a transfer rate of 250Mb/s in one direction and 500Mb/s in full duplex (both directions)

Instead of standard PCI slots you will now have x1PCI Express slots. Each of these slots will have access to an entire lane of the PCI express system. No longer will devices have to share the bandwidth available on the PCI bus. PCI devices on the original PCI bus shared the available bandwidth which was limited to 132MB/s. This shared by all the PCI devices in your system could but a strangle hold on your PCI bandwidth. This new system prevents this from ever being an issue.

Click here for a list of PCI express graphics cards and the latest Prices

The Death of the AGP Bus?

The hype around graphics cards however is slightly different the AGP bus has come to an end. It seems that AGP8x is the last of its genre. To replace this we have a special PCI Express slot. This is simply called a x16 PCI Express slot. This slot used exclusively for graphics cards has 16 PCI express channels bundled into one port. The graphics card plugged into this port will have the ability to utilize all 16 of these channels all for itself. This will allow a maximum transfer rate of 4GB/s in a single direction. The Current AGP Bus can handle a maximum transfer rate of 2GB/s.

The main debate about PCI Express at the moment is between Nvidia and ATI. ATI are implementing there cards with a true native PCI express connection. On the other hand Nvidia are implementing there current technology with an AGP Bridge, to convert PCI Express to AGP signals. This bridge chip is called the High Speed Interconnect (HSI) chip. Its a fully reversible chip will allows AGP GPU's to be run on PCI Express interfaces, and could be utilized to allow the use of PCI Express GPU's on an AGP bus.

As this chip is on the graphics card itself there is no wasted bandwidth on the motherboard, you still get the full bandwidth from the memory to the graphics card. To carry this bandwidth all the way through Nvidia has cranked up the speed of the AGP speed on the card itself to AGP16x. You now have no bandwidth loss. ATI claim the loss is involved with latencies when using a bridge chip during the translation. Now for a reality check. The step up from AGP 4x to 8x did very little for the graphics industry in its current state. 3D games today were not fully utilizing the available bandwidth in AGP 4x. The step up to AGP 8x only allowed for improvement in extreme circumstances anyway. This is not to say that this will always be the case. But my stance on this is by the time you need to the full 16 channels of PCI express Nvidia will be using native PCI Express cards in a whole new generation of graphics technology.

Physical Attributes of PCI Express

The size of the of the PCI express slot depends on the amount of lanes it uses. Because of the nature of the data transfer the more lanes being used the more pins that have to be connected to the motherboard. This determines the size of the slot required. It also means that you can't insert a x1 PCI express card into a x16 PCI slot. Other sizes are available such as x8 and x12 PCI express slots, however it is unlikely that these sizes will turn up in mainstream computers. They will be reserved for special servers.

Above is a picture of a x16 PCI Express slot (top) and a x1 PCI Express slot (bottom)

A x1 PCI Express slot can provide a maximum of 25W of power, you will find that this is more than enough for a single PCI device. A x16 PCI Express slot will have support for 75W of power (bear in mind that these days high powered graphics cards often draw extra power direct from the PSU)