The PC motherboard is a large printed circuit card that usually has the CPU or CPU socket mounted on it along with some peripheral chips (the chipset) . Generally it has mounting fixtures for memory modules and slots for peripheral cards as well. In some low profile designs the expansion cards plug into a riser that allows them to be mounted parallel to the motherboard surface instead of perpendicular. Notebook and other specialized PCs also have motherboards albeit with somewhat different layouts. Almost all motherboards have at least a few configuration jumpers whose setting are usually cryptically explained in text on the motherboard -- at least on modern motherboards.

Over time a number of motherboard form factors have been used in mainstream desktop PCs. The major variations are PC, (XT), AT, and ATX. They differ in size, location of mounting holes, and arrangement of major components. Major vendors also have used a variety of proprietary form factors for motherboards.

The PC and XT used a 22x33cm motherboard usually with 5 to 8 eight bit expansion card slots in the left rear. Power was provided through a pair of six pin connectors plugging into a 12 pin socket. Memory plugged into 36 (generally) 16 pin IC sockets on the motherboard.

The AT form factor was introduced with the PC-AT. It used a larger 30.5x33 motherboard with 8 expansion card slots. Most of these were sixteen bit slots although some boards had fewer slots or had one or more eight bit only slots. Memory was initially provided by IC sockets, however, when 386 CPUs came into use, plug in memory modules -- SIMMs, SIPPs, and later DIMMs were used instead. The slots were generally placed somewhere on the right side of the board -- often buried under the power supply and or drive bays in an assembled PC. Power was provided via the same pair of six pin connectors used on the PC/AT motherboard. Over time, some card slots morphed into MCA bus or VLB and later PCI slots. Cache memory ICs or modules were added -- usually on the front right.

AT mounting screws were generally in different places than PC/XT screws and were somewhat variable in location. It is generally the situation that most AT cases will accept most AT motherboards, but it is often necessary to move or remove mounting brackets(sometimes with a bolt cutter).

As components shrank, so did motherboard form factors. Two major shrinks of the AT motherboard took place:

Baby AT -- 20*23 cm

Half Size AT -- 22*16.5cm

LPX (Low profile, not widely used) -- 23*33cm

In 1995 the ATX form factor was introduced. ATX standardized the location of the CPU (right rear) and replaced the PC power connector with a large ATX connector that carried power and that allowed power to be controlled by motherboard logic. At least five major variations exist

ATX -- 30.5*24.4cm

MiniATX -- 28.4*20.8cm

Micro ATX -- 24.4 * 22.4cm

Flex ATX -- 22.9 * 19.1cm

NLX (Low Profile) 26*22.9cm or 34.55*28.45cm

ITX motherboards are intended to be small motherboards with as many peripherals as possible integrated on the motherboard. Typically they have two PCI slots

ITX -- 21.5*19.1cm

Mini ITX -- 17*17cm

Nano ITX -- 12*12cm

In 2004, Intel announced three new form factors with the CPU moved to the front of the motherboard and PC slots on the rear right. These are designed for improved thermal control

Perhaps surprisingly, BTX motherboards are a bit bigger than ATX. It is asserted that this is because the classical motherboard functions are unlikely to take less space and additional functions take more real estate as do the thermal management optimizations.

BTX -- 32.5*26.6 cm

microBTX -- 26.4*26.6 cm

picoBTX -- 20.3*26.6 cm

Return To Index Copyright 1994-2002 by Donald Kenney.