AT (form factor)

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Computer form factors	PCB Size (mm)
WTX	356×425
AT	350×305
Baby-AT	330×216
BTX	325×266
ATX	305×244
LPX	330×229
NLX	254×228
microATX	244×244
DTX	244×203
FlexATX	229×191
Mini-DTX	203×170
EBX	203×146
microATX (Min.)	171×171
Mini-ITX	170×170
EPIC (Express)	165×115
Nano-ITX	120×120
COM Express	125×95
ETX / XTX	114×95
Pico-ITX	100×72
PC/104(-Plus)	96×90
microETXexpress
nanoETXexpress
mobile-ITX	75×45

In the area of IBM compatible personal computers, the AT form factor referred to the dimensions and layout (form factor) of the motherboard for the IBM AT. Like the IBM PC and IBM XT models before it, many third-party manufcturers produced motherboards compatible with the IBM AT form factor, allowing end users to upgrade their computers for faster processors. The IBM AT became a widely copied design during the booming home computer market in the 1980s. IBM clones made at the time began using AT compatible designs, contributing to its popularity. In the 1990s many computers still used AT and its variants. Since 1997, the AT form factor has been largely supplanted by ATX.

1 Design
2 Variants
3 Power connector
4 External links

[edit] Design

Main article: Industry Standard Architecture

The original AT motherboard, later known as "Full AT", is 12 inches (305 mm) wide and 13.8 inches (350 mm) deep, which means it will not fit in "mini desktop" or "minitower cases". The board's size also means that it takes up space behind the drive bays, making installation of new drives more difficult. The power connectors for AT motherboards are two nearly identical 6-pin cables. Unfortunately, the two power connectors it requires are not easily distinguishable, leading many people to damage their boards when they were improperly connected. When plugged in, the two black wires on each connector must be adjacent to each other, creating a row of 4 black wires (out of the total 12) and a correct connection. Technicians developed mnemonic devices to help assure proper installation, including "black wires together in the middle" and "red and red and you are dead."

[edit] Variants

Baby AT motherboard.

In 1985 IBM introduced Baby AT. Soon after all computer makers abandoned AT for the cheaper and smaller Baby AT, using it for computers from the 286 processors to the first Pentiums. These motherboards have similar mounting hole positions and the same eight card slot locations as those with the AT form factor, but are 2" (51 mm) narrower and marginally shorter. The size (220x330 mm) and flexibility of this kind of motherboard were the key to success of this format. While now obsolete, a few computers are still using it, and modern PC cases are generally backwards compatible to fit Baby AT.

In 1995, Intel introduced ATX, a form factor which gradually replaced older Baby AT motherboards. During the late 1990s, a great majority of boards were either Baby AT or ATX. Many motherboard manufacturers continued making Baby AT over ATX since many computer cases and power supplies in the industry were still compatible with AT boards and not ATX boards. Also, the lack of an eighth slot on ATX motherboards kept it from being used in some servers. After the industry adapted to ATX specifications, it became common to design cases and power supplies to support both Baby AT and ATX motherboards.