ARCHIVED: What are the AT and baby AT form factors?

Form factor is used to describe the physical size and shape of a computer device measured by outside dimensions. It is most often used to describe the size of circuit boards, especially the motherboard and expansion cards.

AT and baby AT form factor

The baby AT form factor was the form factor used by most PC motherboards prior to 1997. The original motherboard for the PC-AT measured 12" by 13". Baby AT motherboards are a little smaller, measuring 8.5" by 13". The baby AT is being replaced by the ATX form factor.

The full-sized AT motherboard, at 12" wide, was too wide to fit into the newer and more common mini-tower and desktop cases being produced, so a narrower baby AT motherboard was introduced that is 8.5" wide. Generally speaking, few full-sized AT motherboards are being produced today and fewer cases are available to hold them. A full tower case will be required to mount a full-sized AT motherboard.

The reduced width of the baby AT motherboard results in less overlap with drive bays, though there may still be some overlap at the front of the case. A recent trend has been to reduce the size of the baby AT board even further, to only 11" or 10" long. This can result in mounting problems when trying to install the motherboard into the case.

AT and baby AT motherboards can be identified by their shape and by the single full-sized keyboard connector soldered onto the board. The serial and parallel port connectors are usually attached using cables that go between the physical connectors mounted on the case and pin connectors located on the motherboard.

The AT and baby AT motherboards have the processor socket and usually the memory sockets located at the front of the motherboard. Long expansion cards and drives were designed to fit over them. The older 386 and 486 processors and 30-pin memory were relatively short, and clearance over them was not an issue. But with the arrival of SIMM and DIMM memory, as well as Pentium and faster processors that require large heat sinks and fans, the combination can often block two or more expansion slots and/or a drive bay. Most newer baby AT motherboards have moved the memory sockets back towards the keyboard connector to try to solve this problem, but the size of the processor still remains a problem. The ATX form factor and case were designed partially to resolve this problem.

For additional information on the AT and baby AT form factors, please see: http://www.pcguide.com/ref/mbsys/mobo/formAT-c.html