Mother of all boards

Here's a teaser for you: off the top of your head, do you know what sort of motherboard your desktop PC contains? Chances are, you'd have to go and look it up. You're not alone. When weighing up the components of a PC they intend to buy, most people first check the processor and clock speed, then the size of the hard drive, and will probably also have a peek at which graphics card is lurking inside. This is a perfectly legitimate way of sizing up a PC, but the motherboard is crucial, too; yet, when it comes to evaluating a system, it rarely gets a look in.

So, what is this mystery component? If you consider the processor as the brain of your machine, the motherboard is the central nervous system, responsible for relaying information between all the internal components. In other words, the motherboard is the electronic glue that pulls together all the parts that make up the PC and presents you with a working machine. Far from exciting, the motherboard is unlikely to make your machine operate any more quickly, but it's essential to have a stable, reliable unit on which to build the rest of the system.

If you're unsure what a motherboard looks like, a quick peek inside your PC will set you straight. It will invariably be the biggest single item in there. It not only plays home to your processor and memory, but all your expansion cards, such as your graphics card, your hard drive and CD-ROM connectors, plus external ports. The motherboard also houses the BIOS (basic input/output system) that controls the simplest configuration of your machine and performs the POST (power on self test) health check when you switch on your machine.

Feeling chipper

All motherboards are not born equal, and you can encounter myriad differences. First and foremost, a motherboard will only support one type of processor, such as a Pentium III, Pentium 4 or Athlon. Different chips have connectors that vary physically from one another. There is no reason for these connectors to be different, but it ensures that you can't plug the wrong chip into the wrong board by accident.

The second way in which motherboards differ is their chipset. If you think of the motherboard as the physical hardware, the chipset is the logic that underlies it. It is the part that dictates how different components actually talk to one another.

Processor development and chipset design go hand in hand - so much so that the chipset is built to support the facilities offered by a certain processor. There used to be a number of chipset vendors on the market, but the consumer market has now been pared down to just three main companies - Intel, AMD and Via. Intel and AMD only produce chipsets for their own processors, while Via makes chipsets for both companies.

The chipset's many functions include the memory controller, EIDE controller, PCI bridge, RTC (real time clock), DMA (direct memory access) controller, IrDA controller, keyboard controller, mouse controller and USB controller. Each of these functions once required dedicated chips, but the introduction of VLSI (very large scale integration) has meant that these functions can now be controlled by only a couple of chips.

A chipset is usually split into two main parts - the north and south bridges. The north bridge takes control of the major functions such as the memory, cache, PCI and AGP connectors, while the south bridge contains the lesser elements such as EIDE, serial and USB controllers.

The parts

Although some motherboards support different features, several key components are present on all current models.

Each will be designed to take a particular type of processor - either slot- or socket-based (almost all are now socket-based); there will be memory module slots for either SDRAM, RDRAM or DDR SDRAM; expansion slots so you can add extras like sound and graphics cards; support for the hard and CD-ROM drives and, finally, connectors for keyboard, mouse and peripherals. It's also quite common for boards to have some built-in basic sound or graphic capabilities.

Back in the days of the 386 and 486, all processors were socketed. The processor itself was square and about 30mm wide, encased in plastic. On the bottom was an array of pins that were approximately 3mm long and pointed downwards. The individual pins were connected to parts of the processor and allowed the chipset to control the chip's operation.

The processor connected to the motherboard via a socket with holes lined up with the pins on the processor. Some older processors were symmetric, so you could accidentally insert them the wrong way round - and fry the chip. Once seated, the processor casing was flush with the socket and required a special tool and a fair degree of dexterity to remove successfully.

Next-generation motherboards solved both these problems. An extra pin was positioned on the inside corner, making the layout asymmetric so that the chip would only fit in the socket one way. The second advancement was the ZIF (zero insertion force) socket, which worked by placing a lever on the side of the socket. To insert the processor you simply lifted up the lever and dropped in the chip. No downward force was necessary - hence zero insertion. On closing the lever, the top half of the socket makes contact with the bottom and the pins line up with the holes. To remove the chip, you just pull up the lever. The most famous ZIF socket is Socket 7, used by later Pentium Is.

Although the ZIF socket worked well enough, when the Pentium II was released it came in a different format, referred to as SECC (single edge contact cartridge). Rather than using a socket, this format connected via a slot similar to a PCI card connector, called Slot 1. One reason for this was that Pentium II moved the cache memory so it could run faster. This meant it sat next to the chip, making the whole cartridge bigger. The SECC housing required two supporting rails to keep it upright in place.

Socket to them

AMD used Socket 7 for its K6 line of chips, but chose a slot format for the Athlon. Slot A was similar to Slot 1, but the processor went in the other way around (so users couldn't inadvertently put an Athlon in a Pentium board, or vice versa). Early PIIIs used the Slot 1 format.

During this time Intel's budget chip, the Celeron, appeared. This started life in the same SECC packaging, but moved to a socket when the Level 2 cache was integrated onto the processor core, relieving the need for the larger cartridge. The new Celeron format was referred to as PPGA (plastic pin grid array) and used 370 pins, so was called Socket 370.

When the Level 2 cache on the PIII was integrated into the core of the processor, Intel also moved this model back to a socket format. Although they utilised the same Socket 370 connector as the Celerons, it wasn't possible to use them in existing Celeron boards.

Newer Pentium IIIs used FCPGA (flip chip pin grid array) packaging - the processor core was on the top of the chip, not the bottom. This placed it in direct contact with the heatsink and increased heat dissipation. The FCPGA packing also rearranged a couple of pin assignments, which prevented them working in older sockets. Modern Socket 370 motherboards accept either Celeron or PIII processors without problem.

AMD has also reverted to sockets with its Thunderbird Athlons, for the same reason - the extra packaging is no longer necessary. Athlons use Socket A, which employs 462 pins. AMD's budget Duron chip also uses the same Socket A format.

SIMM-ply the best

Memory technology has followed the turbulent change of processor design, evolving through many different forms. Until a few years ago 72-pin SIMMs (single inline memory modules) were the norm, seen coupled to Pentium processors.

SIMMs had one major drawback: they had to be installed in matched pairs, so if you wanted 8MB of memory you had to buy two identical 4MB SIMMs and install them next to each other. Next came DIMMs (dual inline memory modules), which only require one module to be used at a time, and are used with current SDRAM and DDR technology.

DIMMs come in different flavours -- PC100 and PC133 for SDRAM, and PC1600 and PC2100 for DDR SDRAM. The numbers relate to the speed at which they can operate (although the DDR numbers use a different measuring system, to make them look better for marketing).

The different speed ratings are due to the FSB (front side bus). Together with the chip's internal multiplier, the FSB determines the processor's overall clock speed. If you use PC133 RAM it will happily run at 100MHz; however, PC100 RAM isn't guaranteed to run at 133MHz. DDR memory uses the same FSB speeds, but sends two data bits on every clock pulse, which is how it got its acronym: DDR stands for double data rate.

A rival memory technology, Rambus, can be found in high-end PCs, including those using Pentium 4s, but is significantly more expensive than DDR and SDRAM DIMMs. AMD recently announced support for DDR RAM in its new chipset. Rambus theoretically offers better performance, but DDR RAM is cheaper and is still faster than PC133 RAM.

What are you driving at

Most motherboards support the EIDE (enhanced integrated drive electronics) standard for hard, CD- and DVD-ROM drives, while others use SCSI (small computer system interface). SCSI is faster and supports more devices, but tends to be more expensive and more difficult to set up. EIDE is more than fast enough for most home and business users.

The EIDE interface evolved from the IDE interface, which supported both CD-ROM and hard drives. This became UDMA (ultra direct memory access), which evolved from DMA and provided faster maximum data rates. UDMA33 was capable of up to 33.3MBps. Each UDMA channel can support two devices, so you could, for example, attach a CD-RW and a DVD-ROM to the same UDMA interface. In general, motherboards have two UDMA channels, though more are possible.

Up to UDMA33, a 40-pin cable was used for all drive connections. The next advance, UDMA66, required an 80-wire cable, but was backwards-compatible in that it also used a 40-pin connector.

To obtain a UDMA66 connection, the motherboard and all devices you attach must support UDMA66. You also need to be using an 80-wire cable. If you had two UDMA66 hard drives attached it would run in UDMA66 mode, but if, say, a non-UDMA66 CD-ROM drive was connected, it would drop back to UDMA33. UDMA100 is the latest development, providing a peak data transfer of 100MBps.

Magic bus

One of the most attractive things about PCs is their upgradeability. To add extra functionality to your machine, you simply need to drop in a suitable card. There have been a number of different expansion slot formats, and until quite recently the main options were the ISA (industry standard architecture) bus and the PCI (peripheral component interconnect) bus.

They were easily recognisable: ISA slots were black, and PCI slots were white. If you look inside a modern PC, you won't find any black ISA slots, as they have been phased out gradually. In addition to PCI slots, there may be an AGP (advanced graphics port) slot. AGP is specifically designed to support the high data transfer rates that complex graphics require. Also, as most new graphics cards are only released in AGP format, you'll need one of these slots if you want to upgrade your graphics card.

Another newcomer is the AMR (audio modem riser) slot, which has been designed with sound card and modem functions in mind. Although it's gaining popularity, most modems and sound cards still come in PCI format.

Top of the form

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

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. There are variations on form factors - for example, MicroATX takes the basic ATX specification, but has fewer expansion slots to allow for smaller cases.

Other motherboard formats exist. AT was the de facto standard before ATX, and NLX is used to create slimline PCs.

Forming attachments

Connectors for the likes of multimedia cards and USB peripherals are attached directly to an ATX motherboard, which makes them far easier to install than on the previous AT form factor (where most of the connectors attached to the motherboard via cables).

The layout is fairly standard and, at the very least, you should find two PS/2 ports (one keyboard, one mouse), two USB ports, one serial and one parallel. Some motherboards also have integrated graphics support, removing the need for a dedicated graphics card, so you'll find either a video connector or a second serial port, as well.

Integrated graphics obviate the need for a separate graphics card but offer poor 3D games performance. If you only want to use office applications then an onboard graphics chip should suffice; diehard gamers, on the other hand, should install a dedicated card.

It's a similar story with the onboard sound processors. You find them welded on to some motherboards, but they offer only basic audio-handling capabilities. If your motherboard has onboard sound, next to the parallel port will be a joystick connector plus headphone, line and microphone sockets. If you want the best sound available, opt for a dedicated card. So you know where to stick your multimedia cards, sockets on the motherboard are colour-coded, making it easy to see what goes where.

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Will Head

PC World

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