Motherboards (continued...)

Form Factors
Form factors define the size, shape, and screw placement on a motherboard. It's usually the technological advancements that have been achieved that allow for these changes so I suppose the form factors also define the technology to some extent.

It's important to understand that none of these standards have to be adhered to (other than maybe spacing between expansion slots). So, if a certain manufacturer decides to build a system board with different screw placement or different dimensions than the standard, it's perfectly acceptable. This board then becomes proprietary and can only be replaced by ordering through the original company. However, if your motherboard adheres to one of the form factors, and you decide to upgrade your motherboard at a future date, or have to replace it for any reason, then all you have to do is buy another motherboard that follows that standard.

PC/XT
When IBM came out with its first Personal Computer (PC), there were no standards and the motherboard tended to be a little on the large size with more space than it really needed. Within a short time, they had developed their Extended Technologies computer (XT), reducing the size of the motherboard to make it more compact and still accept the different circuits and components needed for the system. The XT quickly became a standard for motherboards in many of the clones that were being developed at the time.

AT/baby AT
Computers quickly became more and more powerful with more system memory installed on the board, faster CPUs, and features that required more circuitry and components. IBM had to increase the size of their boards to accept all these components and developed the AT motherboard. At 13.5" X 12", this form factor soon became another standard followed by other manufacturers. Size, screw placement, expansion slot positioning, and even component placement was followed so closely that some motherboard manufacturers were worried about lawsuits. It wasn't uncommon to buy a computer from an established system manufacturer, with full documentation, but no contact information in the motherboard manual, perhaps in fear of copyright infringement.

The AT, however, pretty well became industry standard throughout the 80's and into the early 90's. As technology advanced, circuits and components became smaller and more integrated. Many companies decided to reduce the size of the motherboard again. Because the AT had been standard for so many years they retained the placement of the expansion slots and the screw positioning on a 13" X 8.5 or 9" board. This meant that an AT board could be replaced with a 'baby AT' or 'mini AT' board and still fit in the same case.

The baby AT form factor all but completely replaced the AT and is still available, but has never been made a true standard.

ATX
Because the baby AT form factor was never made a true standard, many companies have taken liberties with different dimensions and design. The ATX is a form factor developed by Intel that closely conforms to the baby AT size. It puts together some of the better ideas, engineering and design to make a standard that is cheaper to develop, allows for better component access, and in some ways is faster and more stable.

The ATX board measures approximately 9.5" X 12" and takes the baby AT board and turns it 90 degrees to put the long edge of the board along the back of the computer case, which provides maximum space for expansion slots and I/O ports.

The different I/O ports, USB connector and the PS/2 keyboard and mouse connectors are stacked or 'layered' and hardwired directly to the motherboard. The absence of a cable connector reduces radio interference as well as production costs. The ATX motherboard also defines the number and placement of mounting holes and uses a different power connection and a different (PS/2) power supply.

The ATX standard also provides for a smaller 'mini ATX' form factor which cuts the size of a board down to 8.2" X 11.2" and removes one row of mounting holes. These boards will fit in a regular ATX style case.