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Beginning in 1996, ATX motherboards became available. Intel developed the ATX specification, and has revised it several times. The broad outlines remain constant across ATX revisions, although some details have changed. ATX was designed to address several deficiencies in the AT and Baby AT form factors, such as

Chapter 2

Altered width/depth ratio Baby AT designs are deeper than they are wide, forcing placement of CPUs or other components in line with the slots. This placement can limit options for the addition of large cards, particularly on CPUs like late-model Pentiums that require large heat sinks with fans. ATX motherboards, by contrast, are wider than they are deep. In an ATX design, the CPU goes to the side of the slots, as shown in Figure 2.1.

Addition of external I/O ports With motherboards universally deploying external I/O ports, the ATX specification includes a standardized location for ports to be built into the motherboard, as shown in Figures 2.1 and 2.7. The keyboard uses a small mini-DIN plug, rather than the larger DIN connector used in AT and ATX motherboards. (Inexpensive adapters allow one type of keyboard to be used on the other type of motherboard.) Mounting external ports directly on the motherboard reduces cable clutter in the computer and improves computer reliability.

Mouse port Keyboard port

Parallel port

Mouse port Keyboard port

Parallel port

USB ports

Serial ports

Figure 2.7

Some ATX motherboards include video or audio connectors in addition to those shown here.

3J O

USB ports

Serial ports

Figure 2.7

Some ATX motherboards include video or audio connectors in addition to those shown here.

• Internal I/O connector locations AT and Baby AT board designers tended to put I/O connectors for floppy, EIDE, and (when present) SCSI adapters anywhere they could. Occasionally these locations produced long and tangled runs of cables to disk drives. ATX boards tend to place these connectors close to where the matching devices end up in a computer's case (see Figure 2.8). This placement minimizes cable clutter.

• Improved power connector ATX motherboards use a single keyed power connector (see Figure 2.9), in contrast to the dual connectors of AT and Baby AT boards. The keyed ATX power connector makes it nearly impossible to destroy a motherboard by

Part i connecting the power supply incorrectly. In addition, ATX boards run on 3.3v, eliminating the need for a voltage regulator. ATX boards also host a series of power-related changes, allowing the computer to power itself off when the OS shuts down, power itself on in response to specified events, go into low-power mode, and so on.

Floppy and EIDE disk connectors

Floppy and EIDE disk connectors

Floppy and EIDE disk devices

Figure 2.8

The design of ATX motherboards and cases places disk cable connectors near the matching disk devices.

Floppy and EIDE disk devices

Figure 2.8

The design of ATX motherboards and cases places disk cable connectors near the matching disk devices.

Figure 2.9

The pins of the ATX power connector are keyed to make it impossible to insert the matching cable incorrectly.

Figure 2.9

The pins of the ATX power connector are keyed to make it impossible to insert the matching cable incorrectly.

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• Cooling changes Although not a motherboard change per se, the ATX specification includes changes to the flow of air through the computer. The new placement of the CPU allows the power supply to blow air over the CPU, theoretically eliminating the need for a separate cooling fan for the CPU. In practice, however, many ATX systems retain a separate CPU cooling fan.

As you might guess, the ATX motherboard design includes enough changes to make it impossible to use an ATX motherboard in an AT case. There are a few hybrid cases available, which can be used with either Baby AT or ATX motherboards, but for the most part, use of an ATX motherboard necessitates use of an ATX case. Therefore, if you have an existing AT or Baby 2

AT system you want to upgrade, you should either look for a Baby AT motherboard or plan to replace your computer's case, as well as the motherboard. 0

Overall, ATX boards represent a substantial improvement over the older Baby AT design. ATX R

b motherboards typically measure approximately 9.6 inches deep by 12.0 inches wide; this width A

limits ATX use in small cases. The smallest case that's practical to use for an ATX mother- D

board is a desktop or mid-tower design. In order to help reduce case size, smaller ATX variants have been developed, including mini-ATX (8.2x11.2 inches), micro-ATX (9.6x9.6 inches), and flex-ATX (9.0x7.5 inches). As with other form factors, these represent maximum sizes. Individual boards can be somewhat smaller in one or both dimensions. The Micro-ATX form factor became quite popular in 1999 among PCs priced at less than $1,000. These smaller ATX variants typically have fewer than the 5-7 usable expansion slots present on full-sized ATX motherboards. Some such boards have only two or three usable slots. This lack of slots is compensated for to some extent by the inclusion of extra functionality on the motherboard. These boards typically include both video and sound features that would require the use of two plugin boards on most full-sized ATX motherboards. Some such motherboards include modems, Ethernet adapters, SCSI host adapters, or other devices on the motherboard, as well.

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