Full Size AT (obsolete), size: , Announced: , Reference:
The PC/XT and AT formats were not compatible. The AT form factor was very popular in the late 1980s, and was the basis of many "clone" manufacturers' units (for compatibility with IBM.)
ATX / Mini ATX / Extended ATX Form Factor, size: , Announced: , Reference: ATX Specification 2.2
The first significant change in case and motherboard design in many years, the ATX form factor was invented by Intel in 1995. ATX and its variants have now become the industry standard in most segments of the market, replacing the former reigning champion, Baby AT. The ATX design has several significant advantages over the older designs, but the enormous installed base of existing Baby AT cases and motherboards has caused the change to ATX to take many years. The ATX case is similar to a Baby AT case except that the holes in the back are altered to fit the changed design of the ATX form factor motherboard, in particular the integrated I/O ports. In most cases a user-replaceable I/O template is used to allow different configurations of I/O ports installed directly on the motherboard. The ATX case also uses a different, ATX power supply, which is also used for NLX form factor cases. Most ATX cases have more features than AT cases because they are newer, and they are sometimes more expensive for the same reason, though this is less common now that AT case production has dropped in volume. Intel has also specified a "Mini ATX" motherboard size, which is slightly smaller than the full-sized ATX specification. These boards use the same ATX form factor power supplies and cases. The main difference is that full ATX motherboards have a maximum size of 12"x9.6", and Mini ATX boards have maximum dimensions of 11.2"x8.2". You may also come across motherboards, particularly high-end models used in performance workstations or servers, that use the "Extended ATX" form factor, also sometimes called "EATX". This form factor is essentially the same as ATX, except that the board can be up to 12"x13" in size. The different form factors in the ATX family are similar, and the biggest differences between motherboards of different ATX-style form factors are dimensions, and the placement of mounting holes. This means that most cases are "downward compatible" with the smaller ATX variants, as long as they have provided appropriate sets of places to put mounting hardware. If a case will support installation of a full-sized ATX motherboard, it may also support a Mini ATX, microATX or FlexATX board as long as the designers of the case have taken the smaller form factor variants into consideration.
Baby AT (obsolete), size: , Announced: , Reference:
Not long after the introduction of the IBM PC/AT and the AT form factor, a smaller version of the AT form factor was created called the "Baby AT" form factor. Baby AT is similar to AT, except that it is smaller in the width dimension. This means that Baby AT power supplies and motherboards will fit into full-sized AT cases, but not vice-versa. Until the rise of ATX, Baby AT form factor PCs dominated the industry. Many Baby AT cases are also supplied with "slimline" LPX power supplies.
BTX size: , Announced: , Reference: All the BTX pdfs
The BTX, or Balanced Technology Extended form factor, unlike its predecessors is not an evolution of a previous form factor but a total break away from the popular and dominating ATX form factor. BTX was developed to take advantage of technologies such as Serial ATA, USB 2.0, and PCI Express. Changes to the layout with the BTX form factor include better component placement for back panel I/O controllers and it is smaller than microATX systems. The BTX form factor provides the industry push to tower size systems with an increased number of system slots. One of the most talked about features of the BTX form factor is that it uses in-line airflow. In the BTX form factor the memory slots and expansion slots have switched places, allowing the main components (processor, chipset, and graphics controller) to use the same airflow which reduces the number of fans needed in the system; thereby reducing noise. To assist in noise reduction BTX system level acoustics have been improved by a reduced air turbulence within the in-line airflow system. Initially there will be three motherboards offered in BTX form factor. The first, picoBTX will offer four mounting holes and one expansion slot, while microBTX will hold seven mounting holes and four expansion slots, and lastly, regularBTX will offer 10 mounting holes and seven expansion slots. The new BTX form factor design is incompatible with ATX, with the exception of being able to use an ATX power supply with BTX boards.
FlexATX, size: , Announced: , Reference:
ITX (obsolete), size: , Announced: , Reference:
FlexATX variation, never produced.
LPX / Mini LPX (obsolete), size: , Announced: , Reference:
One problem with the LPX form factor is that it is only a "pseudo-standard"; it was never formalized into a hard standard, the way for example ATX and NLX have been. Many companies make systems that use slimline cases and LPX-style motherboards and power supplies, but they often differ slightly in size, shape, or other characteristics. This means you cannot expect to move a power supply from say, a Compaq LPX system into a similar-looking Packard Bell system. LPX systems are essentially proprietary.
microATX, size: , Announced: , Reference:
microBTX, size: , Announced: , Reference:
mini-ITX, size: , Announced: , Reference:
Another FlexATX variation.
NLX, size: , Announced: , Reference:
NLX is Intel's proposal for the future of mass-marketed, retail PCs, replacing LPX. One extra advantage of NLX over LPX is that it is a true standard, unlike LPX, making interchangeability of components more likely than it was for the older form factor. NLX seems destined to become of the most popular form factors in the PC world, complementing the ATX "family" of form factors. The NLX specification does not define a new, specific "NLX" power supply form factor. NLX systems are intended to use ATX form factor power supplies.
PC/XT Form Factor, size: , Announced: , Reference:
The PC/XT form factor was replaced by the AT form factor when the IBM PC/AT was released in 1984, though IBM PC/XT cases, as well as large numbers of clones, continued to be found on the market for some time.
PicoBTX, size: , Announced: , Reference:
SSI CEB, size: , Announced: , Reference: http://ssiforum.oaktree.com/ViewUserDocuments.aspx
SSI’s goal is to enable future server market growth by standardizing interfaces between components including boards, chassis, and power supplies and by developing common server hardware elements. At http://ssiforum.oaktree.com/pdfs/SSI%20CEB%20v1-1.pdf is a pdf which has done an incredibly good job of not just specifying the product but also cross referencing other sources and defining terms.
Here are some of the main points which may or may not be right:
- Not compatible ATX power supply You need an SSI power supply. There are no converters.
- Basically the same mounting holes as an ATX but slightly longer 10.5" vs 9.6" but far shorter than the EATX standard at 13". Apparently they needed something slightly larger than an ATX form-factor and knew many existing ATX cases would be able to accomodate a board slightly less than an inch longer so people could keep their cases.
WTX (obsolete), size: , Announced: , Reference: www.wtx.org
In 1998 Intel introduced the new WTX form factor, where the "W" is intended to stand for "workstation". Maximum WTX motherboard size is a whopping 14"x16.75", over double the maximum size of a regular ATX board. The goal of the WTX form factor is to support both current and future high-end motherboard and CPU technologies, and other features in demand by workstation and server users. To that end, the form factor is geared specifically towards flexibility of design; for example, exact mounting hole locations are not prescribed for the case. Instead, the motherboard is designed to mount to a metal plate that comes with it, and the plate installed into the case. You can find detailed specifications and other information about WTX at the WTX Home Page. While the WTX form factor includes a specification for beefy WTX power supplies to power WTX systems, some WTX cases come with ATX power supplies.
