Motherboard

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The motherboard is the main circuit board inside the PC. It holds the CPU and memory, provides expansion slots for peripherals, and, whether directly or indirectly, connects to every part of the PC. The essential motherboard make-up includes the chipset (known as the “glue logic”), some code in ROM and the various wired interconnections between the components know as buses. The chipset is fundamental, and controls how the motherboard interacts with everything else in the system. A good chipset can be more important than the power of CPU or the amount of RAM. The ROM code includes the BIOS, which has user-changeable options for how the motherboard operates with integral and connected devices. The buses are the electrical wires that connect everything together.

MOTHERBOARD FORM FACTORS:

Name
PCB Size (mm)
AT
350×305
ATX
305×244
Baby-AT
330×216
BTX
325×266
COM Express
125×95
DTX
244×203
EBX
203×146
EPIC (Express)
165×115
ESMexpress
125×95
ETX / XTX
114×95
FlexATX
229×191
LPX
330×229
microATX
244×244
microATX (Min.)
171×171
Mini-DTX
203×170
Mini-ITX
170×170
mobile-ITX
75×45
Nano-ITX
120×120
NLX
254×228
PC/104 (-Plus)
96×90
Pico-ITX
100×72
WTX
356×425

Motherboard designs use many different buses to link their various components. For instance, wide, high-speed buses are difficult and expensive to produce. The signals travel at such a rate that even distances of just a few centimetres cause timing problems, while the metal tracks on the circuit board act as miniature radio antennae, transmitting electromagnetic noise that introduces interference with signals elsewhere in the system. For these reasons, design engineers try to keep the fastest buses confined to the smallest area of the motherboard and use slower, more robust buses for other parts.

The original PC motherboard had a minimum of integrated devices, just ports for a keyboard and a cassette deck (for storage). Everything else, including a display adapter and floppy or hard disk controllers, were add-in components, connected via expansion slots.

Over time, more devices were integrated into the motherboard. It was a slow trend initially though, as I/O ports and disk controllers were often mounted on expansion cards even up to 1995. Other components – typically graphics, networking, SCSI and sound – usually remained separate. Many manufacturers have experimented with different levels of integration, building in some or even all of these components. However, there are drawbacks. It’s harder to upgrade the specification if integrated components can’t be removed, and at first highly integrated motherboards often required non-standard cases. Furthermore, replacing a single faulty component may mean buying an entire new motherboard.

Consequently, those parts of the system whose specification changes fastest, particularly RAM and CPU, tend to remain in sockets or slots for easy replacement. Similarly, parts that not all users need, such as SCSI, are usually left out of the base specification to keep costs down. However, it’s now common for sound, video and/or network support to be included in motherboards, particularly as technology miniaturization has allowed greater space within the motherboard form factor and Flash BIOS and EPROM has allowed greater support for updated technologies.

Early PCs used the AT form factor and 12in wide motherboards. The sheer size of an AT motherboard caused problems for upgrading PCs and did not allow use of the increasingly popular slimline desktop cases. These problems were largely addressed by the smaller version of the full AT form factor, the Baby AT, introduced in 1989. Whilst this remains a common form factor, there have been several improvements since.

All designs are open standards and as such don’t require certification. A consequence is that there can be some quite wide variation in design detail between different manufacturers’ motherboards. However, keeping to the standards allows case manufacturers to develop for particular motherboard form factors, a boon for home builders, modders and PC technicians.

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