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AMD aims to unify SFF PCs with DTX spec


Inside the latest effort to standardize small form factor components
— 5:26 PM on October 22, 2007

Ever since we first laid eyes on Shuttle's original XPC, we've been enamored with the concept of small, quiet boxes that still house very capable PCs. You've gotta like the combination of stylish design, power, and convenience that such systems bring to the desktop—or to the countertop, for that matter. For just about that same length of time, though, we've been wishing for a set of standards for small-form-factor systems that would allow DIYers to mix their preferred motherboards with just the right chassis. That desire grew more acute as the popularity of home theater PCs mushroomed, yet building one that's sufficiently quiet, compact, stylish, and powerful remained a maddeningly tedious exercise in component selection and assembly. Today, small form factor PCs have become a staple offering from big PC makers, but they remain a difficult proposition for would-be DIYers and smaller PC vendors.

AMD aims to remedy this situation with a set of standards, known as DTX, that could bring some harmony to the chaotic, inconsistent world of SFF-ready PC components. The firm first introduced the DTX specification early this year, and we now have our Cheeto-stained hands on a prototype of a DTX-compliant reference design system. Keep reading to see what DTX is all about and how the first reference design is shaping up.


AMD's DTX reference design

DTX basics
The key thing to know about DTX is that it's an extension of the existing ATX specification for motherboards and cases, not a wholesale redesign like the ill-fated BTX effort Intel spearheaded a couple of years ago. DTX is also unlike BTX in that it doesn't seek to accommodate the full range of desktop hardware, such as high-wattage processors or the very highest-end GPUs. Instead, AMD's spec seeks the simpler goal of hosting relatively power-efficient hardware in a low-noise chassis. To that end, DTX's peak power envelope for CPUs is 65 watts.

The next thing to know about DTX is that it's an open standard. AMD has proposed the specification and is actively developing and promoting it, but nothing about DTX requires the use of AMD hardware—other than, AMD would probably tell you, the sheer awesomeness of its low-power processors and chipsets with integrated graphics. Obviously, AMD has an interest in creating a healthy ecosystem for smaller PCs given its position as a supplier of such components, but the DTX standard should easily serve the whole of the PC industry, should it catch on.

The heart of DTX, of course, is a set of specifications—actually, two sets of specifications—for building components for teensy-weensy—and teensy weensier—computers. We'll have a look at both sets.


Full DTX in relation to ATX. Source: AMD.

The larger of the two proposed standards is known as Full DTX. As you can see in the diagram above (Don't you love those words? Ah, the sheer entertainment we here at TR provide for you!), a Full DTX mobo is considerably smaller than a standard ATX motherboard—and also smaller than a typical microATX board. AMD has retained the same mounting points as ATX, however, so a DTX board should fit into any ATX case without drama.

At 200 mm wide and 244 mm deep, a Full DTX board leaves room for only two expansion slots, much like a Shuttle XPC. Those slots can be of the PCI or PCIe variety, and DTX provides room for additional expandability via a single, laptop-style XpressCard expansion slot, as well.

AMD estimates Full DTX enclosures will be between eight and 15 liters in volume, and those cases will need to be capable of cooling CPUs with thermal ratings between 45 and 65W. That means no Extreme Edition or FX processors for DTX systems, but it still leaves room for some very high performance CPUs.

Full DTX's motherboard dimensions will allow mobo makers to produce four boards from one standard-sized PCB panel, AMD reckons. They also expect the mobo designs to require only four PCB layers, like most ATX boards. Both of these attributes should make Full DTX appealing for Taiwan, because they should keep manufacturing costs nice and low.


Mini DTX in relation to ATX. Source: AMD.

The second branch of the DTX family tree is known as Mini DTX, and its dimensions are shown above. Mini DTX restricts motherboard depth to 170 mm, but keeps the second expansion slot and accompanying 200 mm width of its DTX sibling. As you've probably guessed, Mini DTX enclosures will be somewhat smaller—AMD estimates between three and eight liters in cubic volume—and won't have the same cooling capacity as Full DTX cases. In fact, AMD has set Mini DTX's upper limit at CPUs with 45W thermal ratings.

The Mini DTX motherboard dimensions should yield six boards per standard PCB panel, but I expect that motherboard designers will have to resort to more than four layers in order to squeeze everything into this space.


Mini-ITX in relation to ATX. Source: AMD.

While we're on the subject, we should take a look at how DTX relates to VIA's Mini-ITX spec, which is popular for embedded systems and other pint-sized-PCs. Well, OK, not exactly pint-sized, but probably less than five liters in cubic volume. At 170 mm by 170 mm, Mini-ITX leaves room for only one expansion slot. AMD estimates Mini-ITX can accommodate CPUs with between 35 and 45W thermal ratings (although VIA's own processors' thermals top out at 20W and range as low as 1W, for what it's worth.)

Because both DTX and Mini-ITX use standard ATX mount points, and because Mini DTX and Mini-ITX are so similar in basic dimensions, AMD believes its DTX push should help establish a chassis standard for Mini-ITX boards, as well.