Hardware Reference
In-Depth Information
The keyboard connector and slot connectors in the full-size AT boards still conformed to
the same specific placement requirements to fit the holes in the XT cases already in use,
but a larger case was 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 applic-
ations.
The important thing to note about the full-size AT systems is that you can always replace
afull-sizeATmotherboardwithaBaby-AT(orXT-size)board,buttheoppositeisnottrue
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 mother-
board 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 po-
sitions 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).
