Hardware Reference
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still required to fit the larger board. Because of the larger size of the board, a full-size AT
motherboard only fits into full-size AT desktop or tower cases. Because these motherboards do not fit
into the smaller Baby-AT or mini-tower cases, and because of advances in component
miniaturization, they are no longer being produced by most motherboard manufacturers—except in
some cases for dual processor server applications.
The important thing to note about the full-size AT systems is that you can always replace a full-size
AT motherboard with a Baby-AT (or XT-size) board, but the opposite is not true unless the case is
large enough to accommodate the full-size AT design.
Baby-AT
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.
Figure 4.4. A late-model Baby-AT motherboard, the Tyan Trinity 100AT (S1590). Photo
courtesy of Tyan Computer Corporation.
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-
 
 
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