AT and Baby AT

Advance Technical Repair of Laptops Motherboard

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One of the first standardized motherboard form factors was used in IBM's PC AT computer, and so the form factor is referred to as the AT form factor. AT motherboards are some of the largest around, measuring up to 12 inches wide and 13.8 inches deep. IBM needed more than one square foot to hold assorted components that today occupy much less space. For instance, RAM in those days was mounted flat on the motherboard, rather than on SIMM, DIMM, or RIMM modules perpendicular to the motherboard.

Over the years, it became possible to shrink components and make more efficient use of space on the motherboard. Therefore, the full 12x13.8-inch space wasn't required. Manufacturers began using a smaller form factor first used with the IBM PC XT computer. Rather than call M

this design the XT form factor, manufacturers called it Baby AT, in order to prevent customers H

from thinking that computers built around it were akin to the less-powerful PC XT computer. ER

Baby AT motherboards can be up to 8.57 inches wide and 13.04 inches deep, although the A

exact dimensions of specific products vary. D

Because of their smaller size, Baby AT motherboards can be used in smaller computer cases, such as mini-tower cases. Many larger cases can accept either full-sized AT or Baby AT motherboards, however.

Most AT and Baby AT motherboards have but a single external connector, a 5-pin DIN for the keyboard. (A few Baby AT motherboards use a mini-DIN connector.) The original PC AT, as well as most AT and Baby AT motherboards until the mid-1990s, relied upon plug-in cards to provide ports for other external devices, such as serial and parallel ports. These plug-in cards had their own external connectors, either directly on the card or attached via ribbon cables and mounted on the computer's case. Later 80486 motherboards, as well as Pentium and later boards, include circuitry for external devices such as serial ports and parallel ports (and, later, mouse ports and USB ports) on the motherboard itself. These motherboards used connectors on the motherboard to provide these services; external connectors were linked to the motherboard via ribbon cables.

AT and Baby AT motherboards used a pair of power connectors, which had to be attached to the case's power supply via cables permanently attached to the power supply (see Figure 2.6). The motherboard itself ran on 5v of current, which caused problems with some CPUs, particularly in the Pentium era, when 3.3v CPU voltage became common. Such motherboards required voltage regulators, which converted 5v to 3.3v. These voltage regulators produced heat, and depending upon the amount of current drawn by the CPU, could overheat and cause unreliable operation.

Part i

Always keep black connectors side-by-side when plugging into the motherboard

Always keep black connectors side-by-side when plugging into the motherboard

P9 P8

Connector Connector

Figure 2.6

AT-style power connectors come in pairs, which must be attached with the black wires together.

P9 P8

Connector Connector

Figure 2.6

AT-style power connectors come in pairs, which must be attached with the black wires together.

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