Over the years, there’s been consistent development in motherboard designs to make them more compact and efficient. Various manufacturers have defined specifications to make motherboards
compatible with other components such as the cabinet, SMPS, and add-on cards. These are design suggestions for the shape and size of the motherboard, location of various components, heat dissipation considerations, power supply design, shielding, etc. These specifications are broadly referred to as the form factor of a motherboard.
Form factors can be broadly divided into three categories–AT, ATX, and NLX. They help motherboard manufacturers in designing motherboards conforming to the same standard. So a user buying a particular standard, say an AT motherboard will go to the market and ask for an AT cabinet.
Each form factor category has several sub-categories that are prefixed by Baby, Mini, and Micro. These are nothing but a set of enhancements provided to the original. The design specifications for each of these can be downloaded from ww.teleport.com/~ffsupport/spec. Other motherboard details can be obtained from www.developer.intel.com/design/motherbd. The motherboard sizes given in parenthesis are broad indicators, and would vary for different manufacturers. Now let’s take a look at some of these.
AT (12” x 13.8”): This form matches the original IBM AT motherboard design. You’ll find these motherboards in very old PCs such as the 286s and 386s. These motherboards can be distinguished mainly by the type of power connector they have. It’s similar to a male plug with flat pins.
Baby AT (13.04” x 8.57”): These evolved from the AT motherboards mainly because of its width and the location of the onboard components on the board. The processor was placed near the expansion slots, which made it difficult for longer cards to fit in. These problems were corrected in the Baby AT form. This form is popular because it can also fit into an existing AT cabinet.
ATX/Mini ATX (12”x 9.6”/11.2”x 8.2”): Intel designed this form factor. It’s essentially a Baby AT
motherboard rotated 90 degrees within the cabinet. This way, the processor is relocated away from the expansion slots, allowing them to hold full-length add-on cards. Another improvement is that it has a new type of power connector with 20 pins instead of the earlier 12. On the SMPS side, there’s a single connector instead of the earlier two, making it easier to insert. That’s not the only improvement. You have to hold the cabinet’s on/off switch for four seconds to power it down. This ensures that a PC isn’t switched off accidentally. ATX power also enables soft power off.
NLX (8.9” x 10-13.6”): This form factor is usually found in a lot of branded desktop machines. In this, the expansion slots are put on a separate riser card plugged into an edge connector on the motherboard. This places the expansion cards parallel to the motherboard, thereby making a more compact design.
After the motherboard shootout this month, we came across a number of interesting market trends. Currently, the most widely available form factors are Baby AT, ATX, and MicroATX. The Baby AT and MicroATX forms are smaller than the ATX with their length and depth being almost equal. Both vary between 8” to 10”. We also noticed that most of them come with both AT and ATX type power connectors to ensure backward compatibility with the earlier AT specifications. So if you have an older cabinet with an AT power supply, then ensure that the motherboard you buy has the right connector. ATX motherboards are much bigger and also expensive.