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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | The Motherboard | Motherboard Form Factors ]

ATX and Mini ATX

The first significant change in case and motherboard design in many years, the ATX form factor was invented by Intel in 1995. After three years, ATX is now finally overtaking AT as the default form factor choice for new systems (although AT remains popular for compatibility with older PCs, with homebuilders, and with some smaller PC shops). Newer Pentium Pro and Pentium II motherboards are the most common users of the ATX style motherboard (not surprisingly, since the Pentium II is the newest processor and uses the newest chipset families.) Intel makes the motherboards for many major name brands, and Intel only uses ATX.

The ATX design has several significant advantages over the older motherboard styles. It addresses many of the annoyances that system builders have had to put up with. As the Baby AT form factor has aged, it has increasingly grown unable to elegantly handle the new requirements of motherboard and chipset design. Since the ATX form factor specifies changes to not just the motherboard, but the case and power supply as well, all of the improvements are examined here:

  • Integrated I/O Port Connectors: Baby AT motherboards use headers which stick up from the board, and a cable that goes from them to the physical serial and parallel port connectors mounted on to the case. The ATX has these connectors soldered directly onto the motherboard. This improvement reduces cost, saves installation time, improves reliability (since the ports can be tested before the motherboard is shipped) and makes the board more standardized.
  • Integrated PS/2 Mouse Connector: On most retail baby AT style motherboards, there is either no PS/2 mouse port, or to get one you need to use a cable from the PS/2 header on the motherboard, just like the serial and parallel ports. (Of course most large OEMs have PS/2 ports built in to their machines, since their boards are custom built in large quantities). ATX motherboards have the PS/2 port built into the motherboard.
  • Reduced Drive Bay Interference: Since the board is essentially "rotated" 90 degrees from the baby AT style, there is much less "overlap" between where the board is and where the drives are. This means easier access to the board, and fewer cooling problems.
  • Reduced Expansion Card Interference: The processor socket/slot and memory sockets are moved from the front of the board to the back right side, near the power supply. This eliminates the clearance problem with baby AT style motherboards and allows full length cards to be used in most (if not all) of the system bus slots.
  • Better Power Supply Connector: The ATX motherboard uses a single 20-pin connector instead of the confusing pair of near-identical 6-pin connectors on the baby AT form factor. You don't have the same risk of blowing up your motherboard by connecting the power cables backwards that most PC homebuilders are familiar with.
  • "Soft Power" Support: The ATX power supply is turned on and off using signalling from the motherboard, not a physical toggle switch. This allows the PC to be turned on and off under software control, allowing much improved power management. For example, with an ATX system you can configure Windows 95 so that it will actually turn the PC off when you tell it to shut down.
  • 3.3V Power Support: The ATX style motherboard has support for 3.3V power from the ATX power supply. This voltage (or lower) is used on almost all newer processors, and this saves cost because the need for voltage regulation to go from 5V to 3.3V is removed.
  • Better Air Flow: The ATX power supply is intended to blow air into the case instead of out of it. This means that air is pushed out of all the small cracks in the PC case instead of being drawn in through them, cutting down on dust accumulation. Further, since the processor socket or slot is on the motherboard right next to the power supply, the power supply fan can be used to cool the processor's heat sink. In many cases, this eliminates the need to use (notoriously unreliable) CPU fans, though the ATX specification now allows for the fan to blow either into or out of the case. See here for more on system air flow and cooling.
  • Improved Design for Upgradability: In part because it is the newest design, the ATX is the choice "for the future". More than that, its design makes upgrading easier because of more efficient access to the components on the motherboard.

Next: LPX and Mini LPX

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