AT (form Factor) - Variants

Variants

In 1985 IBM introduced Baby AT and soon after all computer makers abandoned AT for the cheaper and smaller Baby AT form factor, using it for computers that spanned several generations, from those that used 286 processors to the Pentiums and a limited number of Pentium 2 systems. 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. The development of bigger CPU coolers -- and the fact that they blocked full-length PCI and ISA cards -- spelled the end of Baby AT and was the main impetus for its successor ATX. While the AT standard is now considered to be mostly obsolete, some industrial computers still use it.

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 favored Baby AT over ATX as many computer cases and power supplies in the industry were still designed for 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 shifted to ATX motherboard configurations, it became common to design cases and power supplies to support both Baby AT and ATX motherboards.