Motherboards

PC World Staff (PC World) on 17 June, 2002 17:33

Motherboard form factors

As we told you at the start of this guide, motherboards come in different shapes and sizes. However, to ease the process of designing cases, certain formats, known as form factors, have been standardised.

The form factor refers to the physical layout of the motherboard. It is used to determine the board's feature design, including the physical size and shape of the motherboard, the location of mounting holes and slots, and power supply connectors. You need to match your motherboard form factor with a computer case that supports the particular form factor.

Today, the most common form factor is ATX. The ATX specification not only dictates where the connectors on the back of the motherboard should be (to line up with the holes in the case), but also encompasses details such as the power supply connector. Power supplies are currently available in ATX +12V form factors (20-pins) for most motherboards, but ATX +12V Version 2.01 power supplies are also available for some motherboards (for example, those based on Intel's 925X chipset) that have 24-pin power connectors as well as power connectors for SATA drives and high-end PCI Express graphics cards (such as the Nvidia GeForce 6800 ultra). There are variations on form factors - for example, MicroATX and Mini ATX take the basic ATX specification, but have fewer expansion slots to allow for smaller cases (they can, if desired, still go into larger ATX cases, however).

Prior to ATX, AT was the de facto standard, while NLX was used to create slimline PCs. The biggest difference between the ATX and AT form factors is the power supply mounting configuration. The ATX power supply blows air through the case and across the processor, while the AT form blows air out of the case. The ATX form factor also holds more integrated I/O (Input/Output) and includes a PS2/mouse connector.

Briefly, I/O describes any program, operation or device that transfers data between the devices and a computer. Devices can be divided into those that are input-only, such as keyboards and mouses, and output-only, like printers. The transfer of data to and from the processor to the memory or expansions slots is also referred to as I/O.

AT has been phased out, so you won't find any motherboards based on this form factor on retailers' shelves.

In the future we're likely to see more of a new type of form factor - the BTX (balanced technology extended) form. BTX is designed for better airflow around the components that need it, and to allow PC designers to bring to market more interesting PC designs. BTX is designed to be scalable, from smaller than MicroATX to larger than ATX, in order to support variable numbers of expansion slots.