This document provides an overview of the BabyAT motherboard for personal computers.
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Saturday, October 19, 2013 6:14 PM
The Baby AT Motherboard Form Factor
After IBM released the AT in August 1984, component consolidation allowed subsequent systems to be designed using far fewer chips and requiring much less in the way of motherboard real estate. Therefore, all the additional circuits on the 16-bit AT motherboard could fit into boards using the smaller XT form factor.
The Baby-AT form factor is essentially the same form factor as the original IBM XT motherboard. The only difference is a slight modification in one of the screw hole positions to fit into an AT-style case. These motherboards also have specific placement of the keyboard and slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard 5-pin DIN type connector for the keyboard. Baby-AT motherboards can replace full-size AT motherboards and fit into several case designs. Because of its flexibility, from 1983 into early 1996, the Baby-AT form factor was the most popular motherboard type. Starting in mid-1996, Baby-AT was replaced by the superior ATX motherboard design, which is not directly interchangeable. Figure 4.4 shows the onboard features and layout of a late-model Baby-AT motherboard.
This hardware is considered obsolete and is explained here for reference purposes only. The easiest way to identify a Baby-AT form factor system without opening it is to look at the rear of the case. In a Baby-AT motherboard, the cards plug directly into the board at a 90° angle; in other words, the slots in the case for the cards are perpendicular to the motherboard. Also, the Baby-AT motherboard has only one visible connector directly attached to the board, which is the keyboard connector. Typically, this connector is the full-size 5-pin DIN type connector, although some Baby-AT systems use the smaller 6-pin mini-DIN connector (sometimes called a PS/2-type connector) and might even have a mouse connector. All other connectors are mounted on the case or on card edge brackets and are attached to the motherboard via cables. The keyboard connector is visible through an appropriately placed hole in the case.
Figure 4.4. A late-model Baby-AT motherboard, the Tyan Trinity 100AT (S1590). Photo courtesy of Tyan Computer Corporation.
All Baby-AT boards conform to specific widths and screw hole, slot, and keyboard connector locations, but one thing that can vary is the length of the board. Versions have been built that are smaller than the full 9 inches×13 inches; these are often called mini-AT, micro-AT, or even things such as 2/3-Baby or 1/2-Baby. Even though they might not be the full size, they still bolt directly into the same case as a standard Baby-AT board and can be used as a direct replacement for one.