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Form factors are industry-standardized shapes and layouts that enable motherboards to work with cases and power supplies. A single form factor applies to all three components. All motherboards come in a basic rectangular or square shape, but vary in overall size and in the layout of built-in components (Figure 9-2). You need to install a motherboard in a case designed to fit it, so the ports and slot openings on the back fit correctly.

The power supply and the motherboard need matching connectors, and different form factors define different connections. Given that the term “form factor” applies to the case, motherboard, and power supply—the three parts of the PC most responsible for moving air around inside the PC—the form factor also defines how the air moves around in the case.

To perform motherboard upgrades and provide knowledgeable recommendations to clients, techs need to know their form factors. The PC industry has adopted—and dropped—a number of form factors over the years with such names as AT, ATX, and BTX. Let’s start with the granddaddy of all PC form factors, AT.

AT Form Factor

The AT form factor (Figure 9-3), invented by IBM in the early 1980s, was the predominant form factor for motherboards through the mid-1990s. AT is now obsolete.

Figure 9-2 Typical motherboard

Figure 9-3 AT-style motherboard

The AT motherboard had a few size variations (see Figure 9-4), ranging from large to very large. The original AT motherboard was huge, around 12 inches wide by 13 inches deep. PC technology was new and needed lots of space for the various chips necessary to run the components of the PC.

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NOTE All AT motherboards had a split power socket called P8/P9. You can see the white P8/P9 socket near the keyboard port in Figures 9-3 and 9-4.

Figure 9-4 AT motherboard (bottom) and Baby AT motherboard (top)

The single greatest problem with AT motherboards was the lack of external ports. When PCs were first invented, the only devices plugged into the average PC were a monitor and a keyboard. That’s what the AT was designed to handle—the only dedicated connector on an AT motherboard was the keyboard port (Figure 9-5).

Figure 9-5 Keyboard connector on the back of an AT motherboard

Over the years, the number of devices plugged into the back of the PC has grown tremendously. Your average PC today has a keyboard, a mouse, a printer, some speakers, a monitor, and—if your system’s like mine—four to six USB devices connected to it at any given time. These added components created a demand for a new type of form factor, one with more dedicated connectors for more devices. Many attempts were made to create a new standard form factor. Invariably, these new form factors integrated dedicated connectors for at least the mouse and printer, and many even added connectors for video, sound, and phone lines.

One variation from the AT form factor that enjoyed a degree of success was the slimline form factor. The first slimline form factor was known as LPX (defined in some sources as low profile extended, although there’s some disagreement). It was replaced by the NLX form factor. (NLX apparently stands for nothing, by the way. It’s just a cool grouping of letters.) The LPX and NLX form factors met the demands of the slimline market by providing a central riser slot to enable the insertion of a special riser card (Figure 9-6) or, as it’s sometimes called, a daughterboard. Expansion cards then fit into the riser card horizontally. Combining built-in connections with a riser card enabled manufacturers to produce PCs shorter than 4 inches.

Figure 9-6 Riser card on an older motherboard

The main problem with form factors such as LPX and NLX was their inflexibility. Certainly, no problem occurred with dedicated connections for such devices as mice or printers, but the new form factors also added connectors for such devices as video and sound—devices that were prone to obsolescence, making the motherboard out of date the moment a new type of video or sound card came into popularity.

Essentials