Motherboard form factor

The form factors refers to the physical dimensions (size and shape) as well as certain connector, screw hole, and other positions that dictate into which type of case the board will fit. The more commonly PC motherboard form factors include the following

1  PC and XT

The first popular PC motherboard was the original IBM PC released in August 1981. Fig. shows how this board looked. IBM followed the PC with the XT (eXtended Technology) 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”×13” in size. The XT also eliminated the little used cassette port in the back.

2 Full size AT (Advanced Technology)

This board was 12”×13.8” in size. To accommodate the 16 bit 286 processor and all the necessary support component at the time, IBM needed more room than the original PC/XT size 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. 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. This redesigned form factor is known as XT-286 popularly known as Baby-AT.

3 Baby - AT

Baby –AT is a replacement for the full AT motherboard and sometimes referred to as BAT; the Baby- AT is a motherboard form factor, introduced by IBM 1985 and used with the 8.57”×13.04” deep, which is almost the same as the original IBM XT motherboard with modifications in the screw hole position, to fit into AT-style case. Any case that accepts a full size AT motherboard will also accept a Baby-AT design. The Baby-AT motherboard has only one visible connector directly attached to the board, which is the keyboard connector. Typically this connector is the 5 pin DIN type connector; although some Baby-AT systems use the smaller 6 pin mini DIN connector (PS/2) and might even have a mouse connector.

4 LPX

The LPX (Low Profile eXtended) and mini-LPX form factor board were a semi proprietary

Design by Western Digital. In this form factor the slots are parallel to the main board, enabling the expansion card to install sideways. This allows for a slim or low-profile case design and overall a smaller system than the Baby-AT. The most noticeable feature in LPX is that the expansion slots are mounted on a bus riser card that plugs into the motherboard. Slots are located on one or both sides of the riser card depending on the system and case design.

Another feature of LPX design is the standard placement of connectors on the back of the board. An LPX board has a row of connector for video (VGA 15pin), parallel (25pin) , two serial ports (9 pin each) and mini-DIN PS/2 style mouse and keyboard connector. Some LPX motherboards might have additional connectors for the internal ports, such as network or SCSI adapters.

5 ATX (Advanced Technology eXtended)

ATX is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancement and features. It is physically incompatible with either the previous Baby –AT or LPX design. A different case and power supply are required to match the ATX motherboard. ATX improved on the Baby-AT and LPX motherboard designs in several major areas.

·         Built in double high external I/O connector panel

The rear portion of the motherboard includes a stacked I/O connector area. This enables external connectors to be located directly on the board and negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

·         Single main keyed internal power supply connector

The ATX specification includes a keyed and shrouded main power connector that is easy to plug in and can’t be installed incorrectly. The ATX specification also include two additional optional keyed power connectors called the auxiliary power connector(3.3V and 5V) and the ATX 12V connector for systems that require more power.

·         Relocated CPU and memory

The CPU and memory modules are relocated so they can’t interfere with any bus expansion cards. The CPU and memory are relocated next to the power supply, which is where the primary system fan is located. There is room for a CPU and a heatsink and fan combination.

·         Relocated internal I/O connectors

The internal I/O connectors for the floppy and hard disk drivers are located to be near the drive bay. Therefore internal cables to the drives can be much shorter and accessing the connectors does not require card or drive removal.

·         Improved cooling

The CPU and main memory are designed and positioned to improve overall system cooling. This can decrease the need for separate case or CPU cooling fans. 

·         Low cost to manufacture

The ATX specification eliminates the need of cumbersome wires and additional CPU or chassis cooling fans or onboard 3.3V voltage regulators. This reduced the cost of the motherboard.

6 Micro-ATX

The Micro-ATX is a miniature version of ATX. The reduced size as compared to ATX allows for smaller chassis, motherboard and power supply, thereby reducing the cost of the entire system. The Micro-ATX form factor is also backward compatible with the ATX form factor and can be used in full size ATX cases. The main difference between Micro-ATX and standard or mini-ATX are as follows:

·         Reduced width motherboard

·         Fewer I/O expansion slots

·         Smaller power supply optional

The Micro-ATX motherboard’s maximum size is only 9.6”×9.6”. The Micro-ATX form factor is similar to ATX for compatibility. The similarities include the following:

·         Standard ATX 20 pin power connector

·         Standard ATX I/O panel

·         Mounting holes and dimensions are a subset of ATX

7 Flex-ATX

Flex-ATX’s smaller design is intended to allow a variety of new PC designs, especially extremely inexpensive, smaller consumer-oriented appliance-type system. Some of these designs might not even have expansion slots, allowing expansion only through USB or IEEE 1394/Firewire ports. Flex-ATX defines a board that is upto 9”×7.5” in size, which is the smallest of the ATX family boards.

8 ITX ( Information Technology eXtended )

ITX is a smaller motherboard form factor from VIA technologies, that was first introduced in November 2001 with Mini-ITX. Later versions include the Nano-ITX, Mobile –ITX and the Pico-ITX. With the compact configuration of ITX motherboards, they are typically used in low-cost small setups found in cars, network devices, set-top boxes and other small computers.

9 NLX (New Low profile eXtended)

NLX is similar in initial appearance to LPX, but with numerous improvements designed to enable full integration of the latest technologies. The main characteristic of an NLX system is that the motherboard plug into the riser, unlike LPX where the riser plug into the motherboard. In addition the motherboard in a typical NLX system literally has no internal cables or connectors attached to it. Specific advantages of the NLX form factor include: 

·         Support for all desktop system processor technologies

NLX can handle both AMD and Intel processors.

·         Flexibility in the face of rapidly changing processor technologies

Backplane like flexibility has been built in to the form by allowing a new motherboard to be easily and quickly installed without tearing your entire system to pieces.

·         Support for newer technologies

This includes AGP, USB and memory modules in DIMM or RIMM form.

·         Ease and speed of servicing and repair

NLX systems are the easiest to work on and allow component swaps or other servicing in the shortest amount of time.

10 WTX (Work Station Technology eXtended)

WTX form factor was developed for the mid-range workstation market. WTX motherboards have a maximium width of 14”×16.75”, which is significantly larger than ATX.

11 BTX

Balanced Technology Extended (BTX) is a motherboard form factor specification Intel originally released in September 2003. BTX was designed to address the ever-increasing component power and cooling requirements, as well as enabling improved circuit routing and more flexible chassis designs. BTX is not backward compatible with ATX or other designs. A full-size BTX board is 17% larger than ATX, allowing room for more integrated components onboard. The power supply interface connectors are the same as in the latest ATX 12V specifications. The primary advantages to BTX include

·         Optimized inline component layout and routing - Signals are aligned front to back, allowing connections between components and I/O connectors to run unobstructed.

·         Optimized airflow path - Allows for a condensed system design and an optimized, unobstructed airflow path for efficient system cooling with fewer fans and lower acoustics.

·         Support and retention module (SRM) - Offers mechanical support for heavy heatsinks. It also helps to prevent board flexing or damaging board components and traces during shipping and handling.

·         Scalable board dimensions - Flexible board sizes enable developers to use the same components for a variety of system sizes and configurations.

·         Low-profile options - Component keep-out specifications enable lower profiles, making it easier to design slimline or small form factor systems.

·         Flexible, compatible power supply designs - Connectors are shared with recent ATX designs, smaller, more efficient power supply form factors can be used for small form factor systems, whereas standard ATX 12V power supplies can be used for larger tower configurations.