Design
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. (In IBM's original heavy-gauge steel case, the two 5-1/4" full-height drive bays overhang the front of the motherboard. More precisely, the left bay overhangs the motherboard, while the right bay is subdivided into two half-height bays and additionally extends downward toward the bottom of the chassis, allowing a second full-height fixed disk to be installed below a single half-height drive.)
The power connectors for AT motherboards are two nearly identical 6-pin plugs and sockets. As designed by IBM, the connectors are mechanically keyed so that each can only be inserted in its correct position, but some clone manufacturers cut costs and used unkeyed (interchangeable) connectors. 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, making a row of 4 consecutive black wires (out of the total 12). Technicians developed mnemonic devices to help assure proper installation, including "black wires together in the middle" and "red and red and you are dead."
Read more about this topic: AT (form Factor)
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