The Big Form Factor Guide - Proprietary Form Factors

Dell (circa late 1990s)

Illustration of Dell proprietary relative to other standards

Sometime in the 1990s, Dell began using a non-standard variant of ATX. September 1998 is the date usually quoted, but our review of Dell documentation indicates the same quirks show up in systems back to 1996.

After the introduction of the Pentium 4 in 2000, Dell returned to using a standard ATX form factor (and several years later, BTX). Note that we are not certain that Dell used standard ATX for all of its Pentium 4 systems; certainly the later ones did, but there may be some exceptions among the earliest Pentium 4-based Dells. However, for all Pentium III systems, and potentially all Pentium, Pentium Pro, or Pentium II systems, they used their own proprietary form factor.

These Dells were unique because of the power supply connectors that they used. Dell took 2 standard ATX connectors, the 20-pin "main" connector and the 6-pin "AUX" connector, and created their own non-standard versions of them. The physical connectors are the same, but the wires have been swapped around. This meant, unfortunately, that it was possible to plug a standard ATX power supply into a non-standard Dell, and destroy the motherboard at the first attempt to boot the system. Dell called the 6-pin connector the "3.3" connector, and it carried 3.3 V and ground. They called the 20-pin connector the "POWER" connector, and it carried everything else (5 V, 12 V, -12 V, ground, and the wires used for ATX's "soft" power switch).

Various places on the Internet have provided instructions on how to build an adapter, or made the adapters available for purchase (we link that site in part because their image clearly shows the two connectors on the motherboard). Identifying whether a particular motherboard follows this "standard" is fairly straightforward; if it has what appear to be a regular ATX connector and a 6-pin AUX connector, then it does.

Other than this quirk and some odd connectors for things like the front panel lights, these systems were essentially the same as standard ATX. There is a great deal of disagreement over whether the mounting hole positions and the dimensions of the power supply were standard ATX, and in fact it appears that this is because Dell was not consistent.

Earlier Dells used 10 mounting holes; all of these were usually standard ATX (Dell included the then-required, now-optional hole near the I/O shield, hole B, which is not shown in our drawings) except one. On these boards, the hole known as "G" in the ATX spec (at the bottom-center in our drawings) was often offset slightly (to the left in our drawings). It is not clear why this was done. Even stranger, hole J (top-center in our drawings) is sometimes also offset (to the right in our drawings).

Later (but still early enough to be non-standard) Dells used 7 mounting holes, and were essentially like an ATX motherboard with reduced depth (so they cut off the 3 holes that are right-most in our drawings). They usually used only standard ATX mounting postitions. It appears that Dell began transitioning to the smaller form factor with their Pentium II systems.

The bottom line: If you have a Dell motherboard from the Pentium to Pentium III era, it most likely does not have completely standard mounting positions, nor even mounting positions that match other Dells. And it almost certainly is not compatible with a standard ATX power supply.

Maximum slots: 7
Width: 12 inches (305 mm)
Depth: 9.6 inches (244 mm)
Mounting holes: 7 to 10
Introduced: 1996?
Link: XPS P200s for older, XPS T series for newer

Shuttle "G"

Illustration of Shuttle G relative to other standards

Shuttle's older* form factor for Small Form Factor systems, the "G" form factor is fairly similar to FlexATX or Pico BTX. It has a strange set of mounting holes that are unique to Shuttle, and are somewhat haphazard-looking in their placement. There are an additional 4 holes around the processor used to attach the "ICE" cooling system that we have not illustrated or counted in our table. The ICE screws directly into the case (through the motherboard).

Shuttle systems and barebones systems using the "G", "G2", or any other "G" chassis (such as the SG31G2 or SG31G5) use this form factor. The differences in the various "G"-series chassis are relatively small and mostly cosmetic, with the key exception that systems built on the G5 and subsequent chassis have a 92 mm fan in back, compared to the 80 mm fan on G, G2, and G4 systems (no G3 products were ever released).

The G series all use a proprietary power supply that is similar (but not identical) to a standard "1U" power supply. The power supply is very narrow (roughly the same shape as a common laptop power adapter), and mounted to one side of the case. The connectors are all the same as for the ATX standard. A few G-series models (SD11G5, SS58G2, SK83G, SN85G4, SN95G5 and ST20G5) appear to use a different, slightly more square power supply with a fan on the side facing into the case.

*Shuttle's first SFF systems (the SV "Spacewalker" series) were standard FlexATX with 4 mounting holes and 1 PCI slot. The motherboards measured 175 mm wide and 190 mm deep, and the cases were more cube-like than later XPCs. Unlike the G series, the SV series had a larger (appears to be SFX) power supply mounted at the front of the case. The SV models did not have the "ICE" heat pipe cooling system, just a fan mounted at the top-center of the back of the case.

Maximum slots: 2
Width: 7.25 inches (185 mm)
Depth: 10 inches (254 mm)
Mounting holes: 6
Introduced: 2002
Link: The FN41 is a good example (ZIP of a PDF)

Shuttle "K"

Illustration of Shuttle K relative to other standards

The "K" group (unlike other Shuttle series, these have names that start with the letter K but have no suffix; on other series there is a suffix to indicate form factor when they are sold as a barebones kit) are meant to be the "value" line of XPCs. The K series are a little bit smaller than the G series (the K may stand for "compact", in a weird phonetic way), and significantly cheaper.

The K series is in most ways just a shrunk-down version of the G series. The new form factor is occasionally referred to as the "G7" chassis. The motherboard is smaller in width and depth, the power supply is a bit shorter (and capable of delivering less power), but otherwise it is quite similar. The CPU is in a similar location, it has a similar ICE cooling system, and that ICE is again cooled by a 92 mm fan. The motherboard has just one expansion slot, and all of the mounting hole locations have changed (and are perhaps the most sane of any of Shuttle's layouts).

We should note that while we have called this the "K" (or G7) chassis, Shuttle's forthcoming D series (the first will be the D10) appear to use the same form factor. The difference is that the D series have a touch screen mounted on the front of the case, and are intended for media center applications.

Maximum slots: 1
Width: 6.7 inches (170 mm)
Depth: 9.25 inches (235 mm)
Mounting holes: 6
Introduced: 2008
Link: The K45 is a good example (ZIP of a PDF)

Shuttle "P"

Illustration of Shuttle P relative to other standards

Shuttle's family of "P" form factors (P, P2) are intended for more powerful systems. The form factor was changed quite a bit to accomodate a larger, beefier power supply. The new power supply is similar to an SFX supply (and completely different from the power supply used in the G series) , and mounted at the back of the case, centered. It once again uses standard ATX connectors.

The motherboard was changed fairly dramatically. The processor is now at the front of the case, not the back, and the ICE heatsink for the processor is now much more compact and is built to vent heat out the side of the case, not the back. This "wind tunnel" cooling bears a resemblance to BTX, but even more like BTX is the location of the I/O ports in relation to the expansion slots. Shuttle chose the same location as the one used by BTX, which is the opposite of both ATX and Shuttle's older G form factor. The motherboard is both wider and deeper than the G series, and overall is more square; it is pretty close in dimensions to picoBTX.

The mounting holes are completely different from both the G series and picoBTX. They are more sensible than those used by the G series, but still fairly strange. As for the G series, the processor heatsink has 4 mounting holes that we have excluded.

Cooling is more aggressive than on other Shuttle systems, both because this is intended to be the "performance" series and because at the time of its launch Shuttle was trying to squeeze a more-than-100 W Pentium D and a high-end video card into their tiny case. In addition to the aforementioned "wind tunnel" for the CPU (which has 2 fans) and a fan for the power supply, there are another 2 fans at the top of the back of the case.

Maximum slots: 2
Width: 8 inches (205 mm)
Depth: 11 inches (280 mm)
Mounting holes: 6
Introduced: 2004
Link: The SB95 is a good example (ZIP of a PDF)