The Big Form Factor Guide - Legacy Form Factors
The drumbeat of progress has led to the deaths of a number of form factors. The form factors described on this page have all passed out of mainstream usage, and are described here only for historical interest.
A de facto "standard", AT (short for Advanced Technology) was a product IBM. It was created for the IBM AT, and widely copied by the makers of IBM "clones" of the day. It was enormous compared to many later standards.
AT had many limitations that ultimately led to the creation of ATX. The main power supply connector was a two-piece connector, with two physically-similar connectors that carried different voltages. Connecting them in a "swapped" configuration would kill the motherboard. AT placed the processor in line with some of the expansion slots and near the edge of the board, which meant that large heatsink/fan cooling systems for the processor would collide with large expansion cards or the side of the case. And because of the sheer size of the motherboard, the area reserved for drive bays overlapped the board itself.
|Width:||12 inches (305 mm)|
|Depth:||13.8 inches (351 mm)|
Baby AT was a smaller derivative of AT. It shrank the width of the board significantly, and slightly decreased the maximum depth. In practice, it was not uncommon for boards to have a depth less than the maximum.
Baby AT introduced a smaller form factor for the power supply as well. Due to the smaller motherboard and power supply, Baby AT cases were often significantly smaller than their AT forefathers. Baby AT motherboards and power supplies could be used in AT (and later many ATX) cases, but not vice versa.
After the introduction of Baby AT, some people took to refering to regular AT motherboards as "Full AT". Baby AT displaced AT from the marketplace fairly quickly due to its reduced size, and held a dominant position up until the introduction of ATX.
|Width:||8.5 inches (216 mm)|
|Depth:||13 inches (330 mm)|
A smaller form factor intended for "slimline" cases, LPX (Low Profile Extension) was mainly used by large OEMs looking to build a compact, cheap system. It was originally created by Western Digital and was never really formalized into a true standard. LPX systems were often reliant on proprietary parts and were in general unpleasant to attempt to upgrade.
The AT power connector was reused for LPX.
LPX made use of a riser card that housed the expansion ports. This placed the expansion cards parallel to the motherboard and could reduce the overall size of the system. This configuration, and other aspects of LPX, would ultimately prove to be poorly-suited to processors that required more cooling.
LPX introduced the notion of integrating the input/output ports directly onto the motherboard, which would be used to great effect by ATX and all of its derivatives. Additionally, the power supply dimensions used in ATX are very similar to LPX.
|Width:||up to 9 inches (229 mm)|
|Depth:||up to 13 inches (330 mm)|
|Mounting holes:||Typically 8|
A smaller variant of LPX. This was somewhat closer to a legitimate standard, but implementations could still vary wildly.
|Width:||8 to 9 inches (203 to 229 mm)|
|Depth:||10 to 11 inches (254 to 279 mm)|
|Mounting holes:||Typically 4|
|Link:||Intel Mini-LPX motherboard description|
An evolution of LPX, NLX (New Low Profile Extended) was the creation of several major PC OEMs coordinated by Intel. Like LPX, NLX used a riser card, which held the expansion slots and could also now have internal ports for things like ATA and power. In an effort to maintain some level of sanity in the form factor world, Intel completely reused the ATX power supply design for NLX, including the (then-)new 20-pin connector that replaced AT's two-part main connector.
NLX had a number of quirks, one of which was that cases had to have three sets of mounting holes (labeled sets A, B, and C) in order to support different motherboard sizes. The motherboard only had to have one set of mounting holes, which were all located in a cluster near one of its corners. And while NLX allowed motherboards between 8 and 9 inches in width, motherboards with AGP support had to be 9 inches wide, because the AGP connector had to be placed on the motherboard rather than the riser (presumably due to the tighter design requirements of AGP vs. PCI or ISA).
Where LPX had placed the riser card in the middle of the motherboard (leading to a characteristic support bar across the middle of the case), NLX placed it at the edge and put the card edge on the motherboard and the slot on the riser, instead of the other way around. The standard also suggests (but does not require) that the motherboard be mounted on some sort of rail system, with the riser presumably screwed directly into the case.
This meant that the motherboard could (usually) be slid in and out of the system, with the riser card remaining in place. This was meant to improve servicability of the motherboard, which was the component expected to fail the most frequently (unlike the riser, the motherboard held a lot of complicated electronics, and of course the processor). Ideally, a malfunctioning system could be restored to life by simply sliding out the motherboard and sliding in a new one, letting the technician take his time figuring out what was actually wrong with the motherboard, without inconveniencing the user.
Our image of NLX motherboards shows 3 different lengths (one for each group defined in the standard) plus an example riser (the standard says very little about the riser card). We have added faint lines to show the edge of a minimum-width (8 inch) board.
|Maximum slots:||Typically 4|
|Width:||8 to 9 inches (203 to 229 mm)|
|Depth:||10 to 13.6 inches (254 to 345 mm)|