Motherboard and System devices

Motherboard and System Devices

The motherboard is, in many ways, the most important component in your computer (not the processor, even though the processor gets much more attention.) As mentioned in the Overview, if the processor is the brain of the computer, then the motherboard and its major components (the chipset, BIOS, cache, etc.) are the major systems that this brain uses to control the rest of the computer. Having a good understanding of how the motherboard and its contained subsystems works is probably the most critical part of getting a good understanding of how PCs work in general.
The motherboard plays an important role in the following important aspects of your computer system (notice how many there are here):

• Organization: In one way or another, everything is eventually connected to the motherboard. The way that the motherboard is designed and laid out dictates how the entire computer is going to be organized.
• Control: The motherboard contains the chipset and BIOS program, which between them control most of the data flow within the computer.
• Communication: Almost all communication between the PC and its peripherals, other PCs, and you, the user, goes through the motherboard.
• Processor Support: The motherboard dictates directly your choice of processor for use in the system.
• Peripheral Support: The motherboard determines, in large part, what types of peripherals you can use in your PC. For example, the type of video card your system will use (ISA, VLB, PCI) is dependent on what system buses your motherboard uses.
• Performance: The motherboard is a major determining factor in your system's performance, for two main reasons. First and foremost, the motherboard determines what types of processors, memory, system buses, and hard disk interface speed your system can have, and these components dictate directly your system's performance. Second, the quality of the motherboard circuitry and chipset themselves have an impact on performance.
• Upgradability: The capabilities of your motherboard dictate to what extent you will be able to upgrade your machine. For example, there are some motherboards that will accept regular Pentiums of up to 133 MHz speed only, while others will go to 200 MHz. Obviously, the second one will give you more room to upgrade if you are starting with a P133.

In this chapter of the Reference Guide are several large sections that deal with the various parts that make up a modern motherboard. Many people just consider all of these as part of the motherboard. Physically, the cache, BIOS chip and system buses are on the motherboard, but in terms of operation, I feel it makes more sense to discuss them as "separate but connected". In part this is because the actual location of some of these components could change without affecting their function much. Logically, they are all closely related.

The Motherboard

This section describes the actual motherboard itself, its layout, design and components. The physical board itself varies greatly from PC to PC; two different boards can have very similar performance and capabilities but be laid out totally differently. The large number of motherboard manufacturers contributes to this greatly. The physical motherboard's function is to provide a useful, working place for all of the components of the PC to interface.

Motherboard Form Factors

The form factor of the motherboard describes its general shape, what sorts of cases and power supplies it can use, and its physical organization. For example, a company can make two motherboards that have basically the same functionality but that use a different form factor, and the only real differences will be the physical layout of the board, the position of the components, etc. In fact, many companies do exactly this, they have for example a baby AT version and an ATX version.


AT and Baby AT
Up until recently, the AT and baby AT form factors were the most common form factor in the motherboard world. These two variants differ primarily in width: the older full AT board is 12" wide. This means it won't typically fit into the commonly used "mini" desktop or minitower cases. There are very few new motherboards on the market that use the full AT size. It is fairly common in older machines, 386 class or earlier. One of the major problems with the width of this board (aside from limiting its use in smaller cases) is that a good percentage of the board "overlaps" with the drive bays. This makes installation, troubleshooting and upgrading more difficult.
The Baby AT motherboard was, through 1997, the most common form factor on the market. After three years and a heavy marketing push from Intel, the ATX form factor is now finally overtaking the AT form factor and from here out will be the most popular form factor for new systems. AT and Baby AT are not going anywhere, however, because there are currently just so many baby AT cases, power supplies and motherboards on the market. These will need an upgrade path and I believe that at least some companies will make motherboards for the newer technology in AT form factor for some time, to fill this upgrade market demand.
A Baby AT motherboard is 8.5" wide and nominally 13" long. The reduced width means much less overlap in most cases with the drive bays, although there usually is still some overlap at the front of the case. There are three rows of mounting holes in the board; the first runs along the back of the board where the bus slots and keyboard connector are; the second runs through the middle of the board; and the third runs along the front of the board near where the drives are mounted. One problem with baby AT boards is that many newer ones reduce cost by reducing the size of the board. While the width is quite standard, many newer motherboards are only 11" or even 10" long. This can lead to mounting problems, because the third row of holes on the motherboard won't line up with the row on the case. (Some reduce or skip the third row entirely). Fortunately, it is almost always possible to solidly mount the motherboard using only the first two rows of holes, and then using stubbed spacers for the third row. See the Motherboard Physical Installation Procedure for more perspective on these issues.
Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard connector soldered onto the board. The serial and parallel port connectors are almost always attached using cables that go between the physical connectors mounted on the case, and pin "headers" located on the motherboard.
The AT and Baby AT form factors put the processor socket(s)/slot(s) and memory sockets at the front of the motherboard, and long expansion cards were designed to extend over them. When this form factor was designed, over ten years ago, this worked fine: processors and memory chips were small and put directly onto the motherboard, and clearance wasn't an issue. However, now we have memory in SIMM/DIMM sockets, not directly inserted onto the motherboard, and we have larger processors that need big heat sinks and fans mounted on them. Since the processor is still often in the same place, the result can be that the processor+heat sink+fan combination often blocks as many as three of the expansion slots on the motherboard! Most newer Baby AT style motherboards have moved the SIMM or DIMM sockets out of the way, but the processor remains a problem. ATX was designed in part to solve this issue.


ATX and Mini ATX
The first significant change in case and motherboard design in many years, the ATX form factor was invented by Intel in 1995. After three years, ATX is now finally overtaking AT as the default form factor choice for new systems (although AT remains popular for compatibility with older PCs, with homebuilders, and with some smaller PC shops). Newer Pentium Pro and Pentium II motherboards are the most common users of the ATX style motherboard (not surprisingly, since the Pentium II is the newest processor and uses the newest chipset families.) Intel makes the motherboards for many major name brands, and Intel only uses ATX.
The ATX design has several significant advantages over the older motherboard styles. It addresses many of the annoyances that system builders have had to put up with. As the Baby AT form factor has aged, it has increasingly grown unable to elegantly handle the new requirements of motherboard and chipset design. Since the ATX form factor specifies changes to not just the motherboard, but the case and power supply as well, all of the improvements are examined here:

• Integrated I/O Port Connectors: Baby AT motherboards use headers which stick up from the board, and a cable that goes from them to the physical serial and parallel port connectors mounted on to the case. The ATX has these connectors soldered directly onto the motherboard. This improvement reduces cost, saves installation time, improves reliability (since the ports can be tested before the motherboard is shipped) and makes the board more standardized.
• Integrated PS/2 Mouse Connector: On most retail baby AT style motherboards, there is either no PS/2 mouse port, or to get one you need to use a cable from the PS/2 header on the motherboard, just like the serial and parallel ports. (Of course most large OEMs have PS/2 ports built in to their machines, since their boards are custom built in large quantities). ATX motherboards have the PS/2 port built into the motherboard.
• Reduced Drive Bay Interference: Since the board is essentially "rotated" 90 degrees from the baby AT style, there is much less "overlap" between where the board is and where the drives are. This means easier access to the board, and fewer cooling problems.
• Reduced Expansion Card Interference: The processor socket/slot and memory sockets are moved from the front of the board to the back right side, near the power supply. This eliminates the clearance problem with baby AT style motherboards and allows full length cards to be used in most (if not all) of the system bus slots.
• Better Power Supply Connector: The ATX motherboard uses a single 20-pin connector instead of the confusing pair of near-identical 6-pin connectors on the baby AT form factor. You don't have the same risk of blowing up your motherboard by connecting the power cables backwards that most PC homebuilders are familiar with.
• "Soft Power" Support: The ATX power supply is turned on and off using signalling from the motherboard, not a physical toggle switch. This allows the PC to be turned on and off under software control, allowing much improved power management. For example, with an ATX system you can configure Windows 95 so that it will actually turn the PC off when you tell it to shut down.
• 3.3V Power Support: The ATX style motherboard has support for 3.3V power from the ATX power supply. This voltage (or lower) is used on almost all newer processors, and this saves cost because the need for voltage regulation to go from 5V to 3.3V is removed.
• Better Air Flow: The ATX power supply is intended to blow air into the case instead of out of it. This means that air is pushed out of all the small cracks in the PC case instead of being drawn in through them, cutting down on dust accumulation. Further, since the processor socket or slot is on the motherboard right next to the power supply, the power supply fan can be used to cool the processor's heat sink. In many cases, this eliminates the need to use (notoriously unreliable) CPU fans, though the ATX specification now allows for the fan to blow either into or out of the case. See here for more on system air flow and cooling.
• Improved Design for Upgradability: In part because it is the newest design, the ATX is the choice "for the future". More than that, its design makes upgrading easier because of more efficient access to the components on the motherboard.

LPX and Mini LPX
Conventionally used in mass-produced "name brand" retail systems, the LPX motherboard form factor goes into the small Slimline or "low profile" cases typically found on these sorts of desktop systems. The primary design goal behind the LPX form factor is reducing space usage (and cost). This can be seen in its most distinguishing feature: the riser card that is used to hold expansion slots.
Instead of having the expansion cards go into system bus slots on the motherboard, like on the AT or ATX motherboards, LPX form factor motherboards put the system bus on a riser card that plugs into the motherboard. Then, the expansion cards plug into the riser card; usually, a maximum of just three. This means that the expansion cards are parallel to the plane of the motherboard. This allows the height of the case to be greatly reduced, since the height of the expansion cards is the main reason full-sized desktop cases are as tall as they are. The problem is that you are limited to only two or three expansion slots!
LPX form factor motherboards also often come with video display adapter cards built into the motherboard. If the card built in is of good quality, this can save the manufacturer money and provide the user with a good quality display. However, if the user wants to upgrade to a new video card, this can cause a problem unless the integrated video can be disabled. LPX motherboards also usually come with serial, parallel and mouse connectors attached to them, like ATX.
While the LPX form factor can be used by a manufacturer to save money and space in the construction of a custom product, these systems suffer from non-standardization, poor expandability, poor upgradability, poor cooling and difficulty of use for the do-it-yourselfer. They are not recommended for the homebuilder, but if you are upgrading one of these systems, you may not have many alternatives.


Comparison of Form Factors
This table is a summary comparison of the sizes of the various motherboard form factors, and compatibility factors.

Style	Width	Depth	Where Found	Match to Case and Power Supply
Full AT	12"	11-13"	Very Old PCs	Full AT, Full Tower
Baby AT	8.5"	10-13"	Older PCs	All but Slimline, ATX
ATX	12"	9.6"	Newer PCs	ATX
Mini ATX	11.2"	8.2"	Newer PCs	ATX
LPX	9"	11-13"	Older Retail PCs	Slimline
Mini LPX	8-9"	10-11"	Older Retail PCs	Slimline
NLX	8-9"	10-13.6"	Newer Retail PCs	Slimline

Note: Some ATX cases will accept baby AT form factor motherboards.