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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
- 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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