Comparison of computer form factors

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This article compares the IBM compatible personal computer motherboard form factors – that is, the different sizes and specific or de-facto standards of major system components. In all cases, at least the motherboard footprint, mounting, and connections are specified. Less frequently, dimensions for cases and power supplies is also standardized. Power supply voltages and current requirements may also be given.

The specifications are considered a form factor (as opposed to a model) when enough information is available so that FRU-level parts can be sourced from more than one OEM.

There are actually many computer form factors. These can generally be classified according to category of application (especially in embedded systems) or by architecture (e.g. CHRP). However, this comparison is limited to ISA (IBM compatible) PC architectures, compatible evolutions of it (legacy-free), or form factors that have evolved to accommodate ISA-compatible CPUs (e.g. -ITX and ETX).

Overview of form factors

Pictorial comparison of some common computer form factors.

A PC motherboard is the main circuit board within a typical desktop computer, laptop or server. It has a number of functions of which the main ones are:

• As a central backbone to which all other modular parts (CPU, RAM, hard drives etc) can be attached as required to create a modern computer
• To accept (on many motherboards) different components (in particular CPU and PCI cards) for the purposes of customization.
• To distribute power to many of the PC components
• To electronically co-ordinate the operation of these, and interface all of these with one another.

As new generations of components have been developed, the standards of motherboards have changed too - for example with AGP being introduced, and more recently PCI Express. However the basic standardized size and layout of motherboard have changed much more slowly, and are controlled by their own standards. This is helped by the fact that in many ways, the list of components a motherboard must include changes far slower than the components themselves. For example, north bridge controllers have changed many times since their original introduction, with many manufacturers bringing out their own versions, but in terms of form factor standards, the requirement to allow for a north bridge has remained fairly static for many years.

Although it is a slower process, form factors do evolve regularly in response to changing demands. The original PC standard (AT) was superseded in 1995 by the current industry standard ATX, which still dictates the size and design of the motherboard in most modern PCs. The latest update to the ATX standard was released in 2004. A divergent standard by chipset manufacturer VIA called EPIA (aka -ITX, and not to be confused with EPIC) is based upon smaller form factors and its own standards.

Differences between form factors are most apparent in terms of their intended market sector, and involve variations in size, design compromises and typical features. Most modern computers have very similar requirements, so form factor differences tend to be based upon subsets and supersets of these. For example, a desktop computer may require more sockets for maximal flexibility and many optional connectors and other features on-board, whereas a computer to be used in a multimedia system may need to be optimized for heat and size, with additional plug-in cards being less common. The smallest motherboards may sacrifice CPU flexibility in favor of a fixed manufacturer's choice.

Comparisons

Tabular information

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Graphical comparison of physical sizes

This image compares the sizes of common form factors to ISO 216 paper sizes (e.g. A4) (Sizes are in mm):

Visual examples of different form factors

Example images of different form factors

ATX (Abit KT7)

mini-ITX (VIA EPIA 5000AG)

Pico-ITX (VIA EPIA PX10000G)

PC/104 and EBX

PC/104 is an embedded computer standard which defines both a form factor and computer bus. PC/104 is intended for embedded computing environments. Single board computers built to this form factor are often sold by COTS vendors, which benefits users who want a customized rugged system, without months of design and paper work.

The PC/104 form factor was standardized by the PC/104 Consortium in 1992.[3] An IEEE standard corresponding to PC/104 was drafted as IEEE P996.1, but never ratified.

The 5.75 x 8.0 in. Embedded Board eXpandable (EBX) specification, which was derived from Ampro's proprietary Little Board form-factor, resulted from a collaboration between Ampro and Motorola Computer Group.

As compared with PC/104 modules, these larger (but still reasonably embeddable) SBCs tend to have everything of a full PC on them, including application oriented interfaces like audio, analog, or digital I/O in many cases. Also it's much easier to fit Pentium CPUs -- whereas it's a tight squeeze (or expensive) to do so on a PC/104 SBC. Typically, EBX SBCs contain: the CPU; upgradeable RAM subassemblies (e.g. DIMM); Flash memory for solid state disk; multiple USB, serial, and parallel ports; onboard expansion via a PC/104 module stack; off-board expansion via ISA and/or PCI buses (from the PC/104 connectors); networking interface (typically Ethernet); and video (typically CRT, LCD, and TV).

Mini PC

Mini PC is a PC form factor very close in size to an external CD or DVD drive.

Examples

• AOpen mini PC
• Apple Computer's Mac mini

References

The official Intel Form factors website containing form factor descriptions

1. ^ image

See also

• Motherboard
• Small form factor
• Hard-disk-drive form factors
• 19-inch rack