Thursday, January 17, 2008
Form Factors of the Motherboard
NLX (New Low Profile Extended) was a form factor proposed by Intel and developed jointly with IBM, DEC, and other vendors for low profile, low cost, mass-marketed retail PCs. Release 1.2 was finalized in March 1997 and release 1.8 was finalized in April 1999. NLX was similar in overall design to LPX, including a riser card and a low-profile slimline case. It was modernized and updated to allow support for the latest technologies while keeping costs down and fixing the main problems with LPX.
Many slimline systems that were formerly designed to fit the LPX form factor were modified to fit NLX. NLX is a true standard, unlike LPX, making interchangeability of components easier than it was for the older form factor. IBM, Gateway, and NEC produced a fair number of NLX computers in the late 1990s, primarily for Socket 370 (Pentium II-III and Celeron), but NLX never enjoyed the widespread acceptance that LPX had. Most importantly, one of the largest PC manufacturers, Dell decided against using NLX and created their own proprietary motherboards for use in their slimline systems. Although many of these computers and motherboards are still available secondhand, new production has essentially ceased, and in the slimline and small form factor market, NLX has been superseded by the Micro-ATX, FlexATX, and Mini-ITX form factors
LPX (form factor)
LPX (Low Profile eXtension), originally developed by Western Digital, was a loosely defined motherboard format (form factor) widely used in the 1990s.
There was never any official LPX specification, but the design normally featured the main I/O ports mounted on the back of the motherboard (something that was later adopted by the ATX form factor), and a riser card in the center of the motherboard, on which the PCI and ISA slots were mounted. Due to the lack of standardised specification, riser cards were seldom compatible from one motherboard design to another, much less one manufacturer to another. The internal PSU connector was of the same type used in the AT form factor; most LPX cases were compatible with AT power supplies, though some used models that were smaller than standard, and usually manufacturer-specific.
The specification was very popular in the early-mid 90's, and briefly displaced the AT form factor as the most commonly used. However, the release of the Pentium II in 1997 highlighted the flaws of the format, as a good airflow was important in Pentium II systems, owing to the relatively high heat dispersal requirements of the processor. LPX systems suffered a restricted airflow due to the centrally placed riser cards. The introduction of the AGP format further complicated matters, as the design not only increased the pincount on riser cards, but it limited most cards to one AGP, one PCI and one ISA slot, which was too restrictive for most users. Some lower-quality LPX boards didn't even feature a real AGP slot, but simply used a physical AGP slot and connected it to the PCI bus. This was seldom noticed however, as many "AGP" graphics cards of the time were in fact PCI cards internally, and did not take advantage of the features offered by AGP.
LPX was phased out around 1998. NLX was the intended successor, though many manufacturers chose MicroATX or proprietary motherboard formats instead.
ATX (form factor)
The ATX (for Advanced Technology Extended) form factor was created by Intel in 1995. It was the first big change in computer case and motherboard design in many years. ATX overtook AT completely as the default form factor for new systems. ATX addressed many of the AT form factor's annoyances that had frustrated system builders. Other standards for smaller boards (including microATX, FlexATX and mini-ITX) usually keep the basic rear layout but reduce the size of the board and the number of expansion slot positions. In 2003, Intel announced the new BTX standard, intended as a replacement for ATX. As of January 2007 the ATX form factor remains the industry standard for do-it-yourselfers; BTX has however made inroads into pre-made systems, being adopted by computer makers like Dell, Gateway, and HP.
The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.2, released in 2004.
A full size ATX board is 12" wide by 9.6" deep (305 mm x 244 mm). This allows many ATX form factor chassis to accept microATX boards as well.
ATX was originally designed with the power supply drawing air into the case and exhausting it down onto the motherboard. The plan was to deliver cool air directly to the CPU's and power regulation circuitry's location, which was usually at the top of the motherboard in ATX designs. This was not particularly useful for a variety of reasons. Early ATX systems simply didn't have processors or components with thermal output that required special cooling considerations. Later ATX systems with significantly greater heat output would not be aided in cooling by a power supply delivering its often significantly heated exhaust into the case. As a result, the ATX specification was changed to make PSU airflow optional.
With the introduction of the Pentium 4, the standard 20-pin ATX power connector was deemed inadequate to supply increasing electrical load requirements. The standard was revised with an extra 4-pin, 12-volt connector. This was later adopted by Athlon XP and Athlon 64 systems. Various high-end systems may have other forms of supplemental power connections.
Because video card power demands have dramatically increased over the 2000s, some high-end graphics cards have power demands that exceed AGP or PCIe slot capabilities. For these cards, supplementary power was delivered through a standard power connector like those used for hard drives or floppy drives. PCI Express-based video cards manufactured after 2004 typically use a standard 6 or 8-pin PCIe power connector directly from the PSU.
Because the ATX PSU uses the motherboard's power switch, turning on the power in situations that do not utilize an ATX motherboard is possible by shorting the green wire from the ATX connector to any black wire on the connector (or ground). This allows re-use of an old PC power supply for tasks other than powering a PC, but one must be careful to observe the minimum load requirements of the PSU.
The ATX form factor has had five, main power supply designs throughout its lifetime:
ATX - 20 pin connector (Used through Pentium III and early Athlon XP)
WTX - 24 pin connector (Pentium II and III, Xeon and Athlon MP)
AMD GES - 24 pin main connector, 8 pin secondary connector (some dual-processor Athlon)
EPS12V - 24 pin main connector, 8 pin secondary connector, optional 4 pin tertiary connector (Xeon and Opteron) defined in SSI specification
ATX12V - 20 pin main connector, 4 pin secondary connector, 8 pin tertiary connector (Pentium 4 and mid/late Athlon XP & Athlon 64)
ATX12V 1.3 - guidance for the -5 volt feed was removed. This was only used by legacy ISA add-in cards
ATX12V 2.0 - 24 pin main connector, 4 pin secondary connector (Pentium 4, Core 2 Duo, and Athlon 64 with PCI Express)
ATX12V 2.2 - One 20/24-pin connector, one ATX12V 4 pin connector. Many power supply manufacturers include a 4 plus 4 pin, or 8 to 4 pin secondary connector instead, which can also be used as the secondary EPS12V connector.
AT and Baby AT
Up until recently, the AT and baby AT form factors were the most common form factor in the motherboard world. These two variants differ primarily in width: the older full AT board is 12" wide. This means it won't typically fit into the commonly used "mini" desktop or minitower cases. There are very few new motherboards on the market that use the full AT size. It is fairly common in older machines, 386 class or earlier. One of the major problems with the width of this board (aside from limiting its use in smaller cases) is that a good percentage of the board "overlaps" with the drive bays. This makes installation, troubleshooting and upgrading more difficult.
The Baby AT motherboard was, through 1997, the most common form factor on the market. After three years and a heavy marketing push from Intel, the ATX form factor is now finally overtaking the AT form factor and from here out will be the most popular form factor for new systems. AT and Baby AT are not going anywhere, however, because there are currently just so many baby AT cases, power supplies and motherboards on the market. These will need an upgrade path and I believe that at least some companies will make motherboards for the newer technology in AT form factor for some time, to fill this upgrade market demand.
Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard connector soldered onto the board. The serial and parallel port connectors are almost always attached using cables that go between the physical connectors mounted on the case, and pin "headers" located on the motherboard.
The AT and Baby AT form factors put the processor socket(s)/slot(s) and memory sockets at the front of the motherboard, and long expansion cards were designed to extend over them. When this form factor was designed, over ten years ago, this worked fine: processors and memory chips were small and put directly onto the motherboard, and clearance wasn't an issue. However, now we have memory in SIMM/DIMM sockets, not directly inserted onto the motherboard, and we have larger processors that need big heat sinks and fans mounted on them. Since the processor is still often in the same place, the result can be that the processor+heat sink+fan combination often blocks as many as three of the expansion slots on the motherboard! Most newer Baby AT style motherboards have moved the SIMM or DIMM sockets out of the way, but the processor remains a problem. ATX was designed in part to solve this issue.
The AT form factor was the first modern form factor to be widely used. AT (Advanced Technology) was released in 1984 by IBM. Unlike the PC and XT form factors that preceded it, AT became a widely used design as a result of the booming home computer market in the 1980s. IBM clones made at the time began using AT compatible designs, contributing to its popularity. In the 1990s many computers still used AT and its variants, but ATX has been the most popular standard since 1997.
Main article: Industry Standard Architecture
The original AT motherboard, later known as "Full AT", is 12 inches (305 mm) wide and 13.8 inches (350 mm) deep, which means it will not fit in "mini desktop" or "minitower cases". The board's size also means that it takes up space behind the drive bays, making installation of new drives more difficult. The power connectors for AT motherboards are two nearly identical 6-pin cords. Unfortunately, the two power connectors it requires are not easily distinguishable, leading many people to damage their boards when they were misconnected. However, technicians need only remember the phrase "black wires together in the middle" or "red and red and you are dead" to prevent this. When plugged in, the two black wires on each connector must be adjacent to each other, creating a row of 4 black cords (out of the total 12) and a correct connection.