System (Motherboard) Form Factors
The form factor of the motherboard describes its general shape, what sorts of cases and power supplies it can use, and its physical organization. For example, a company can make two motherboards that have basically the same functionality but that use a different form factor, and the only real differences will be the physical layout of the board, the position of the components, etc. In fact, many companies do exactly this, they have for example a baby AT version and an ATX version.
The AT and baby AT motherboards are currently the most common form factor in the motherboard world. These two variants differ primarily in width: the older full AT board is 12" wide. This means it won't typically fit into the commonly used "mini" desktop or minitower cases. There are relatively few new motherboards on the market that use the full AT size. It is very common in older machines, 386 class or earlier. One of the major problems with the width of this board (aside from limiting its use in smaller cases) is that a good percentage of the board "overlaps" with the drive bays. This makes installation, troubleshooting and upgrading more difficult.
The Baby AT motherboard is currently the most common form factor on the market. This is changing as acceptance of the ATX form factor grows (due in no small part to a heavy push from its inventor, Intel) but there are currently just so many baby AT cases, power supplies and motherboards on the market, that it will take a long time before baby AT goes away.
A Baby AT motherboard is 8.5" wide and nominally 13" long. The reduced width means much less overlap in most cases with the drive bays, although there usually is still some overlap at the front of the case. There are three rows of mounting holes in the board; the first runs along the back of the board where the bus slots and keyboard connector are; the second runs through the middle of the board; and the third runs along the front of the board near where the drives are mounted. One problem with baby AT boards is that many newer ones reduce cost by reducing the size of the board. While the width is quite standard, many newer motherboards are only 11" or even 10" long. This can lead to mounting problems, because the third row of holes on the motherboard won't line up with the row on the case. Fortunately, it is almost always possible to solidly mount the motherboard using only the first two rows of holes, and then using stubbed spacers for the third row.
Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard connector soldered onto the board. The serial and parallel port connectors are almost always attached using cables that go between the physical connectors mounted on the case, and pin "headers" located on the motherboard.
The AT and Baby AT form factors put the processor socket(s)/slot(s) and memory sockets at the front of the motherboard, and long expansion cards were designed to extend over them. When this form factor was designed, this worked fine: processors and memory chips were small and put directly onto the motherboard, and clearance wasn't an issue. However, now we have memory in SIMM/DIMM sockets, not directly inserted onto the motherboard, and we have larger processors that need big heat sinks and fans mounted on them. Since the processor is still often in the same place, the result can be that the processor+heat sink+fan combination often blocks as many as three of the expansion slots on the motherboard! Most newer Baby AT style motherboards have moved the SIMM or DIMM sockets out of the way, but the processor remains a problem.
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 starting to make headway as the default form factor choice for new systems (although AT remains popular for compatibility with older PCs, with homebuilders, and with 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:
1. 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.
2. 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.
3. 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.
4. 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.
5. "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.
6. 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 a voltage regulator to go from 5V to 3.3V is removed.
7. Better Air Flow:
The ATX power supply blows 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. See here for more on system air flow and cooling.
8. 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.
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