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Sunday, February 21, 2010

Best Of Tom’s Hardware: Beginner’s Guide To Motherboard Selection

Best Of Tom’s Hardware: Beginner’s Guide To Motherboard Selection 

We were all beginners once, but it's easy for experts to forget how much they didn't know at the outset of their journey. So, even as Tom's Hardware constantly publishes motherboard comparisons, it's easy to overlook the fact that many newcomers lack the requisite knowledge to take advantage of the advice we give. If ever you've found yourself scratching your head after reading about a layout critique, a certain form factor, or a particular chipset, today's update was written just for you.
What goes into choosing a motherboard? Certainly the support for your CPU of choice is key, and Tom's Hardware is there again with everything from low-power mobile processors to 200+ watt overclocked monsters. Still, a motherboard is far more than the component a CPU plugs into!
A somewhat common worst-case scenario for first-time builders is to spend hundreds of dollars in parts, only to find that some won’t fit together. Less common is when parts that fit together don't work together. But the most frequent problem new builders face is an inappropriate component selection that limits the performance of their high-priced build, making it feel like a less-expensive machine. We'll try to help you avoid that awkward moment, when you realize this board won't fit into that enclosure.
Choosing parts that fit and work well together requires consideration of motherboard size, socket type, and chipset features. Getting the best performance involves intricacies like memory configuration and graphics support. Ultimate functionality requires consideration of onboard devices and/or additional card slots.
That seems like a lot of data to take into consideration before even making a purchase, and with over a dozen brands offering hundreds of options, nobody said it would be easy. However, a little general knowledge and a few reviews can take the guesswork out of motherboard selection so you can narrow the market down to a small number of "best matched" models.

Component Overview

Vast PCB real estate allows motherboard manufacturers to pack many features onto a full-ATX motherboard, resulting in a bevy of associated components to consider:

Retained from our 2006 article, this Socket 775 motherboard still represents the layout of typical high-end products, allowing anyone who’s unfamiliar with onboard components to use it as a guide.
Layout features and details depend on the motherboard’s target market, price point, and a few underlying technologies. Main power comes to the above example through a 24-pin extended ATX (EPS) power connector (12), with the CPU core powered separately via an eight-pin CPU power (EPS12v/ATX12v) connector (13). 
Another input common on motherboards with dual interfaces for graphics (10) is a standard four-pin Molex receptacle (14), which offers additional power to the PCI Express (PCIe) x16 slots (10).
Most of today's motherboards have one or more PCIe x16 slots (10) to support the latest 3D graphics solutions, though some use fewer lanes to connect the cards at reduced bandwidth. Entry-level motherboard models have no graphics expansion slots at all and are designed to work primarily from on-board graphics.
A three-phase voltage regulator (15) is easily identified by three distinct groups of components that work in parallel. Modern motherboards typically use a greater number of lower-amperage power phases to provide smoother power under load, a design that also allows unneeded components to be disabled under low-load conditions for enhanced energy savings. Differences in component capacity make it impossible to determine the quality of a motherboard based on its phase count, as even our recent extreme-range motherboard shootout revealed a 12-phase motherboard with a higher rated capacity than its 35-phase competitor.
Motherboards with digital voltage regulators usually lack the distinct groups of components seen above, forcing builders to rely on reviews or manufacturer documentation to determine voltage regulator specifics.

 Layout Considerations

With the components defined, we can now take a closer look at how they affect layout and how layout is affected by them. The Asus motherboard below has several strong features and a few drawbacks that will put things into perspective.

A primary layout consideration is the amount of clearance that graphics cards have relative to other devices. The example above puts the uppermost PCIe x16 slot in the second slot position (see the blue slot), providing a small amount of space between the back of a long graphics card and the DIMM latches to ease the replacement of memory with a graphics card installed. Increasing that space could have been accomplished by moving the RAM towards the motherboard's upper edge, shown on the right in this photo, and Asus has more recently released motherboards that have no latches on the lower end, shown on the left side of the DIMM slots above.
The lower PCIe slot on the above board is a different matter. Behind it are four SATA connectors, two of which (the red ones) may be blocked by graphics card coolers on long cards, such as ATI’s Radeon HD 5850 and Nvidia’s GeForce GTX 260. Long cards would further block access to the blue ATA/100 connector, except that it has wisely been turned 90 degrees.
Most current “enthusiast” motherboards have now turned the SATA connectors in the same direction as the ATA connector above, so that it must be plugged in from the front edge. While this allows extra-long expansion cards to be installed into nearly any slot, some traditional case designs have a hard drive cage that blocks access to forward-facing ports. While most “enthusiast” market cases have been redesigned to allow SATA cables to pass around the hard drive cage, case reviews should be able to clear up any doubts.
The above motherboard stands out having slots in all seven slot positions, whereas many alternatives support only five or six cards. Asus accomplished this without crowding the DIMM slots, by putting a PCIe x1 slot above the uppermost x16 slot (see the small white connector next to the blue PCIE x16 slot). Typical PCIe x1 cards are usually short enough to avoid crowding DIMM latches, although the oversized chipset heat sinks of many motherboards further restrict card length.
The top edge is preferred for the ATX12V/EPS12V connector because it allows the cable to be pulled past the CPU cooler without blocking or touching its fan. This design also allows the cable to be pulled up behind the motherboard tray in cases that have the power supply at the bottom, and many manufacturers have redesigned their high-amperage power supplies with longer cables to accommodate this configuration.
The larger 20- or 24-pin ATX/EPS power connector should always be placed at the front edge of modern motherboards to allow easy access in cases that have the power supply at the top or bottom, without blocking the CPU cooler or any expansion slots. Some entry-level motherboards have placed this connector between the CPU socket and rear-panel ports, a design that never gets the stamp of approval from Tom’s Hardware, regardless of the motherboard’s other qualities.
Because Serial ATA (SATA) cables now connect both optical drives and hard drives or solid-state disks, the connectors should be placed near the mid-point of a motherboard’s front edge. The motherboard photographed above was designed for an era when SATA was reserved primarily for hard drives, and instead placed its outdated Ultra ATA connector appropriately for the upper bays of tower cases.
The ideal location for front-panel audio connections is behind the rear-panel audio jacks, to ease cable routing for mid- or top-mounted front panel ports. Because this portion of the board is usually crowded, the area in front of the slots becomes an acceptable location for front-panel USB and IEEE 1394/FireWire connectors. Front-panel connections should never be mounted in the lower rear corner of the board, as many cases use cables that simply won't reach that far. Yet because so many otherwise-exemplary motherboards have inconvenient front-panel connector placement, the final decision of whether or not to use a particular motherboard can be based on a thorough examination of the chosen case.
A final layout consideration is fan connector number and placement. The motherboard pictured above provides a perfect minimum configuration, its CPU fan connector near the DIMM slots, exhaust fan header near the audio ports, intake fan connector in the front lower corner, and another fan connector near the northbridge. Two intake and exhaust fan headers are preferable, and connections for any side and top fans should also be considered. Adapters may be used to connect additional fans directly to the power supply, although this method removes the motherboard’s ability to control fan speed.
Admittedly, we are critical of layout and positioning issues, but our goal is to indicate possible problems, not to eliminate any particular product from further consideration.

Form Factors

ATX was designed to address three major shortcomings of the earlier AT form factor, and it offers a few minor improvements as well. First, a designated portion of the board for the CPU socket keeps it out of the way of long cards, where AT boards had the CPU mounted behind or in place of card slots. Second, the inclusion of a port panel on the motherboard itself negates the need for slot brackets to break out such common items as USB, eSATA, or audio ports. Third, a cooling path from the lower front to the upper rear of the case vents hot air through the power supply and/or an exhaust fan. All three major improvements are centered on splitting the board between the slot and CPU area.

Most significant among the minor improvements was the addition of power-on through the motherboard. This allowed the system to turn itself off at shutdown and made possible such features as wake-on-ring (using a modem), wake-on-LAN (using a network adapter), timed power up/power down, and keyboard power-on hot-buttons.
ATX derivatives are based on the same CPU section, so that smaller boards are able to fit into larger cases if desired. ATX standards include microATX and FlexATX. Most Shuttle-style PC cubes (often called SFF for Shuttle Form Factor or Small Form Factor) use a two-slot variation of the FlexATX form factor reduced to approximately eight inches (later standardized by AMD as the exactly 8" DTX form factor), and VIA further shortened its mini-ITX form factor to 6.75" by reducing the maximum slot count to one. ATX size specifications are based on fractional inches.

The image above compares the maximum size and maximum number of slots allowed on various ATX-based form factors, with dashed lines indicating how the mounting holes in smaller boards still align with those of larger cases.

Choosing The Right Processor Socket

While many builders prefer the latest CPU technology, two older CPU sockets have remained on the market as manufacturers produced less-expensive CPU models to support even the lowest budgets. The oldest of these, Intel’s LGA 775, is being phased out, while AMD’s Socket AM2, which is newer, is expected to follow. AMD’s Socket AM2+ and AM3 and Intel’s LGA 1366 and 1156 are well-established replacements for the former CPU interfaces. We arranged brief descriptions in reverse chronological order.

Intel LGA 1156

Supporting Intel’s Core i3-, i5-, and 800-series i7 microprocessors, LGA 1156 motherboards connect two channels of DDR3 memory and 16 full-speed (5.0 Gb/s) PCIe 2.0 lanes directly to the processor. Because all northbridge functions, including memory and primary PCIe control, have been moved onto the processor, additional PCIe connections are available only through the "southbridge" component that remains on the motherboard itself, a part Intel has renamed as its Platform Controller Hub (PCH). Using the slower DMI interface (traditional for Intel northbridge-to-southbridge connections), the PCH provides only 2.5 Gb/s per pathway, and is therefore unsuitable for high-bandwidth applications such as graphics cards.

Because of its PCIe limitations, LGA 1156 is generally best-suited for users who require very few high-bandwidth expansion cards, including some users who rely almost exclusively on CPU performance. An acceptable workaround for the PCIe limitation has also been found for certain motherboard and high-performance graphics configurations.

AMD Socket AM3

Socket AM3 motherboards are nearly identical to similarly-named AM2+ models, but have DDR3 memory slots. Because Socket AM3 processors support both DDR2 and DDR3, AM3 processor users can choose between AM3 and AM2+ motherboards depending on memory preference. Current prices for DDR2 and DDR3 4GB dual-channel kits are similar, but we expect DDR3 will eventually be the better value as production of DDR2 declines.

Intel LGA 1366

Supporting 900-series Core i7 processors, LGA 1366 provides the motherboard with three memory channels and a high-bandwidth QPI interface for its chipset. Originally home to several quad-core processors, most early LGA 1366 motherboards will also support six-core models via a BIOS update. Yet, the most common reason buyers choose LGA 1366 isn’t for the processors it supports, but for the high number of PCIe lanes supported by its accompanying X58 Express chipset. Thus, LGA 1366 is the best choice for users who need both top CPU performance and added support for high-bandwidth expansion cards.

 AMD Socket AM2+

AM2+ motherboards bridge the gap between the company’s DDR2 and DDR3 products by supporting Socket AM3, AM2+, and AM2 processors. To enable cross-compatibility, AM2+ motherboards support the higher-speed HyperTransport 3.0 interconnect of AM2+ and AM3 processors as well as the slower HyperTransport interconnect of Socket AM2 processors. Because Socket AM2 and AM2+ processors support DDR2 exclusively, all AM2+ motherboards have DDR2 sockets.
Because of its flexibility, Socket AM2+ is the best solution for anyone who wants to build an AMD-based personal computer using DDR2 memory. 

AMD Socket AM2

AMD’s Socket AM2 processors are compatible with newer AM2+ motherboards, which in turn are also compatible with AM2+ and AM3 processors. Because of this, anyone concerned about upgrade or service replacement capability should skip this generation of motherboards entirely, even if they’ve chosen an AM2 processor.

Intel LGA 775

Intel's first “pinless socket” for desktop processors, its LGA 775 originally addressed the issue of high-speed Pentium 4 processors drawing too much power by increasing the number of connections. Intel credits the design for eliminating the lead-based solder formerly used for attaching socket pins. Unfortunately, flexible contacts within the socket can be very fragile and repeated rebuilds have left many testers with dead boards.

Currently being phased out in favor of its later LGA 1156 interface, the use of low-cost processors is the only reason to choose LGA 775 over its replacement. New LGA 775 motherboard models have supported Core 2-series processors since 2006, although newer processor models often require a revised motherboard BIOS just to get the system to boot. Most new purchases are not affected by these compatibility issues, but buyers should check the motherboard manufacturer’s CPU support list if the question of compatibility arises.

Choosing The Right Chipset

The gateway between a processor and other components is a set of interface controllers generically called the chipset. Traditional chipsets include a northbridge with memory controller and graphics card interface and a southbridge with slower expansion card interfaces and various peripheral, storage, and communications controllers. The only remaining mainstream platforms that still fit the traditional description are based on Intel’s LGA 775.
AMD was first to remove the memory controller from its chipsets and put it on the CPU in its Socket 754 launch back in 2003. Intel followed the move with its LGA 1366 interface in 2008. The company further integrated with its LGA 1156 platform by moving the primary graphics interface controller (PCIe 2.0) to the CPU, and then released dual-core LGA 1156 processors with the on-board graphics engine that was previously found in value-oriented northbridges.

AMD Northbridge Options
AMD’s 700-series chipsets provide a multitude of options for both discrete (no onboard graphics) and integrated graphics customers. Northbridge products include:
  • 790FX, with 42 PCIe 2.0 links, the best match for multiple graphics cards.
  • 790X, a lower-cost 26-lane version of the 790FX that supports a single graphics card with 16 lanes or two cards with eight lanes per card.
  • 790GX, an integrated-graphics version of the 790X that supports discrete graphics, integrated DirectX 10.0 graphics, and combinations of AMD discrete cards with integrated graphics.
  • 785G, an updated version of the 780G that supports DirectX 10.1 and HDMI 1.3.
  • 780G, a lower-cost version of the 790GX that supports a single AMD graphics card and integrated graphics simultaneously for enhanced multi-monitor support.
  • 780V, a lower-cost version of the 780G that supports fewer graphics features.
  • 770, a version of the 780G that has no integrated graphics and supports a single graphics card in true x16 mode. An additional graphics card can be hosted at reduced bandwidth by using four of the chipset’s x1 pathways.
  • 760G, a version of the 780G with a reduced-performance graphics engine that supports integrated and discrete graphics, but not simultaneously.
  • Older models such as the 740G that do not support the higher-speed HyperTransport connection of AM2+ and AM3 processors.

AMD Southbridge Options

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