AT Form Factor

In 1984 IBM introduced the IBM PC/AT, "AT" standing for "advanced technology", an abbreviation whose use still survives to this day in some contexts. Very similar in overall physical design to the PC and XT models that preceded it, the power supply in these units was increased in size and changed slightly in shape, establishing it as a distinct form factor. Whereas "clone" manufacturers made a few PC/XT units compatible with the IBM PC and XT, it was with the AT that the PC world really began to explode. Many different manufacturers began creating AT-compatible systems and with them, AT form factor power supplies. The original AT power supply provided 192 W--a respectable figure, especially for the time, which represented a tripling of the original PC's power supply output just three years later. It is used in AT form factor cases and with AT or Baby AT form factor motherboards. It has the same motherboard and drive connectors as the PC/XT form factor.

Diagram of the side and rear views of an AT desktop form factor power supply,
with approximate dimensions. The fan here has been moved to the middle
of the back of the supply, and the voltage selector switch has been placed
near the power cord receptacle, a much more sensible location. Note the
distinctive "missing corner" at the bottom right of the rear view, which was
created to allow part of the power supply to "overhang" part of the motherboard.

Original image � PC Power & Cooling, Inc.
Image used with permission.

The AT form factor was the first to introduce tower-style cases and systems to the PC world. The desktop power supply and tower power supply were internally the same. The only difference was in the on/off switch. The desktop used the same red toggle switch as the PC and XT, while the tower introduced the first remote power switch in the PC world. The control wires for the switch were passed through the same hole in the front of the power supply case that was used for the motherboard and drive connector bundle.

Diagram of the side and rear views of an AT tower form factor power supply,
with approximate dimensions. The only change is the removal of the toggle
switch in favor of a remote power switch (not shown).

Original image � PC Power & Cooling, Inc.
Image used with permission.

The AT form factor is now obsolete of course, though many of these systems remain in use. After only a few years, the AT form factor was quickly dethroned by its smaller successor, the Baby AT form factor, due to the advantages of the latter's smaller size, so far fewer AT systems than Baby AT systems are found today.