Computer Beginner Guide: XT and AT Motherboard

XT and AT Motherboard

The first popular PC motherboard was, of course, the original IBM PC released in August 1981. IBM followed the PC with the XT motherboard in March 1983, which had the same size and shape as the PC board but had eight slots instead of five. Both the IBM PC and XT motherboards were 9''x13'' in size.

Also, the slots were spaced 0.8'' apart in the XT instead of 1'' apart as in the PC. The XT also eliminated the little used cassette port in the back, which was supposed to be used to save BASIC programs on cassette tape instead of the much more expensive (at the time) floppy drive.

The minor differences in the slot positions and the deleted cassette connector on the back required a minor redesign of the case. In essence, the XT was a mildly enhanced PC, with a motherboard that was the same overall size and shape, used the same processor, and came in a case that was identical except for slot bracketry and the lack of a hole for the cassette port.

Eventually, the XT motherboard design became very popular, and many other PC motherboard manufacturers of the day copied IBM's XT design and produced similar boards.

Full-Size AT

The full-size AT motherboard form factor matches the original IBM AT motherboard design. This allows for a very large board of up to 12'' wide by 13.8'' deep. The full-size AT board first debuted in August 1984, when IBM introduced the Personal Computer AT (advanced technology).

To accommodate the 16-bit 286 processor and all the necessary support components at the time, IBM needed more room than the original PC/XT-sized boards could provide. So for the AT, IBM increased the size of the motherboard but retained the same screw hole and connector positions of the XT design.

To accomplish this, IBM essentially started with a PC/XT-sized board and extended it in two directions. A little more than a year after being introduced, the appearance of chipsets and other circuit consolidation allowed the same motherboard functionality to be built using fewer chips, so the board was redesigned to make it slightly smaller.

Then, it was redesigned again as IBM shrank the board down to XT-size in a system it called the XT-286 (introduced in September 1986). The XT-286 board was virtually identical in size and shape to the original XT, a form factor which would later be known as Baby-AT.

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 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.

IBM was one of the first to use the smaller boards when it introduced a system called the XT-286 in September 1986. Unfortunately the "XT" designation in the name of that system caused a lot of confusion, and many people did not want to buy a system they thought used older and slower technology.

Sales of the XT-286 were dismal. By this time, other companies had also developed XT-size AT class systems. However, they decided that rather than calling these boards XT-size, which seemed to make people think they were 8-bit designs, they would refer to them as "Baby-AT" designs.

The intention was to make people understand that these new boards had AT technology in a smaller form factor and were not souped-up versions of older technology as was seemingly implied by IBM's XT-286 moniker. Thus, 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 5pin DIN type connector for the keyboard.

Baby-AT motherboards can be used to replace full-size AT motherboards and will 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.

Most systems sold since 1996 have used the improved ATX, micro-ATX, or NLX design, and Baby-AT is getting harder and harder to come by. Older Baby-AT motherboards have the same general layout but lack advanced features, such as USB connectors, DIMM memory sockets, and the AGP slot.

Any case that accepts a full-size AT motherboard will also accept a Baby-AT design. PC motherboards using the Baby-AT design have been manufactured to use virtually any processor from the original 8088 to the Pentium III or Athlon, although the pickings are slim where the newer processors are concerned.

As such, systems with Baby-AT motherboards were the original upgradeable systems. Because any Baby-AT motherboard can be replaced with any other Baby-AT motherboard, this is an interchangeable design. Even though the Baby-AT design is now obsolete, ATX carries on its philosophy of interchangeability.

Figure below shows a more modern Baby-AT motherboard, which includes USB compatibility, SIMM and DIMM sockets, and even a supplemental ATX power supply connection.

A late-model Baby-AT motherboard, the Tyan Trinity 100AT (S1590).

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.

Baby-AT boards all 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''x13'' size; 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.