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Last updated: 12/20/99

A PC's form-factor essentially defines the shape of its motherboard, how the motherboard attaches to the case, and the power supply connectors and electrical characteristics. There are several widely available motherboard form-factors: Baby AT, ATX, microATX and NLX. The microATX and older NLX are  "standards" mainly for low-end, all-in-one computers.  In my opinion, the Baby AT and ATX are today's "generic" motherboards.

The Baby AT form-factor has been around for long time as far as PC history is concerned. It evolved from the IBM AT computer motherboard, which was originally designed around the Intel 80286 processor in the early 1980's . The AT motherboard was a very large motherboard by today's standards. As the supporting logic chips ("glue logic") on the motherboard became more integrated and motherboard speeds increased,--requiring shorter distances (PC board traces) between logic chips--the requirement for PC board real-estate shrunk. The board evolved and became narrower. And many of them have continued to shrink, growing shorter from front to back.

The ATX form factor is a further evolution of the Baby-AT. It is essentially a Baby-AT rotated 90 degrees, but with expansion board slots still running from the back to the front of the case--picture the slots standing still and the board rotating under them. This allows relocation of the principle elements of the motherboard to better locations. The CPU has been moved from the front of the Baby-AT motherboard to the right side of the ATX motherboard and next to the power supply. This allows the design of power supply can directly cool the CPU, solves the problem of the CPU blocking expansion board slots and/or interfering with floppy disk and hard disk drives. It also permits placement of the hard disk and floppy disk interfaces at the front of the board and, thus, shorter cables to the drives. The back side of the board has a panel to which connectors for serial and parallel ports are fastened and soldered to the board. These connectors are soldered to the board instead of led to the case exterior using cables, as is the case with the Baby-AT. Radio frequency emissions from Baby-AT cables are thus eliminated. All of these improvements were supposed to reduce the cost of the ATX form-factor compared to the Baby-AT. Such is not the case, yet. ATX motherboards, cases, and power supplies generally cost little more retail than Baby-AT equivalents.

The Baby-AT power supply has two connectors going to the motherboard. If you plug them in backwards, you may damage the motherboard ("black in the middle, you're ok; red, you're dead"). The ATX power supply has one connector. It can only be plugged-in one way. Also, ATX power supplies can turn on by themselves if you 'touch certain certain motherboard components or connections with metallic objects'. I believe I have plugged-in Baby-AT, AT, and XT connectors backwards four times in the 10-11 years I have been working on them. I do forget to un-plug the power cord more often than that

You cannot use an ATX board to upgrade a computer with a Baby-AT case. You can use a Baby-AT motherboard and power supply in many ATX cases-they have mounting holes for both. Many recent Baby-AT motherboards have power connectors for both Baby-AT and ATX power supplies.

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