PC Interfaces 101

AGP - Accelerated Graphics Port

AGP-Slot with a retaining clip for the graphics card

Most of the graphics cards in PCs today use the Accelerated Graphics Port (AGP) as their standard interface. Fewer systems (mostly older ones) use PCI connected graphics cards instead. Coming on strong is PCI Express (PCIe) as the quick new kid on the graphics block. Warning: PCI Express is a serial bus, whereas PCI (without the Express suffix) is a parallel bus. PCI and PCI Express are completely different and shouldn't be switched!

An AGP graphics card (above) compared to a PCI Express graphics card (below)

Workstation motherboards have an AGP Pro slot, which supplies additional power to energy-gobbling OpenGL graphics cards. It is designed to work with normal mainstream graphics cards as well. However, AGP Pro has not become widely accepted. Instead, power-hungry cards are equipped with a separate power source, either through a Molex or peripheral plug.

Supplementary power for a graphics card with a 4- or 6-pin peripheral power connector (typical on modern PSUs)

Supplementary power for a graphics card with a Molex connector

Within AGP there are four bandwidth classes:

Those who like to experiment with hardware should also be aware that there are two voltages for these interfaces as well. AGP 1X and 2X require 3.3 V, but AGP 4X and 8X require only 1.5 V; in addition, there's a Universal AGP card type that goes both ways. In any case, AGP slots all have built-in notches to prevent devices of the wrong type from being seated in incompatible slots.

Upper card with notch on the left for AGP 3.3 V. Middle: universal card with two notches (one for AGP 3.3 V, the other for AGP 1.5 V). Bottom card with notch on the right for AGP 1.5 V.