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Accelerated Graphics Port
Boards of the accelerated graphics port (AGP) design follow a physical standard derived from that of PCI, much as the AGP electrical interface is an offshoot of PCI. The AGP design assumes that the 3D accelerator chip and frame buffer will have a home on an expansion board rather than the motherboard, although nothing about the interface design precludes putting all the circuitry on the motherboard. In a typical computer, an expansion slot supporting the AGP connection fits on the motherboard like a PCI slot, with the same spacing and dimensional requirements. Although a given computer or motherboard may omit a PCI slot in favor of an AGP connection, AGP is electrically separate from the PCI slots and does not reduce the number of slots that may be connected to a single bridge.
The AGP circuit board design is built around a conventional PCI card, at least for use in full-size computers—those matching (or approximating) the ATX design. For these systems, the original AGP standard provides for the two sizes of card. A full-size AGP board measures 4.2 inches (107 millimeters) tall and 12.28 inches (312 millimeters) long. Short cards measure only 6.875 inches (175 millimeters) long. Figure 30.5 illustrates these principal dimensions.
For low-profile computers that match the NLX standards, the revised AGP 2.0 standard provides a special, low-slung card form factor that allows the AGP-based graphics adapter to plug into the motherboard rather than the riser that accommodates other expansion boards. Figure 30.6 shows the dimensions of the NLX-style AGP board.
The slot spacing for AGP expansion boards in ATX computers is the same as for PCI and ISA boards, 0.8 inches. AGP boards in such systems use the same retaining bracket design as ordinary PCI boards. The NLX design, however, requires a specialb bracket, as shown in Figure 30.7.
Although the AGP slot bin a computer uses an edge connector, it is an entirely different connector from those used in PCI slots. This physical difference ensures against inserting an expansion board in an inappropriate slot. Other aspects of AGP further constrained the design of the connector. The chosen design interleaves contacts at two depths inside the connector with the mating fingers on the edge connector arranged in a flip-flop pattern, as shown in Figure 30.8.
The interleaved contact fingers on the AGP card make hot-plugging AGP boards impossible. That is, you should not slide an AGP board into its slot when the power is on to your computer. When you slide a board into a slot, the contact fingers closest to the edge of the board are apt to wipe across the connector contacts for the deeper contacts. Even a quick wipe can send weird, even damaging signals through the card and your computer. Intel recommends that you slide AGP boards into their slots as straight as you can without rocking them back and forth to seat them in the connector. Even when the power is off, the side-to-side wiping action can damage the miniaturized contacts in the AGP connector.
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