In the era of

IBM compatible An IBM PC compatible is any personal computer that is hardware- and software-compatible with the IBM Personal Computer (IBM PC) and its subsequent models. Like the original IBM PC, an IBM PC–compatible computer uses an x86-based central pro ...

personal computer A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...

s, the AT form factor comprises the dimensions and layout ( form factor) of the

motherboard A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...

for the IBM AT. Baby AT motherboards are slightly smaller, measuring 8.5" by 13". Like the

IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...

and IBM XT models before it, many third-party manufacturers produced motherboards compatible with the IBM AT form factor, allowing end users to

upgrade Upgrading is the process of replacing a product with a newer version of the same product. In computing and consumer electronics, an upgrade is generally a replacement of hardware, software or firmware with a newer or better version, in order to ...

their computers for faster processors. The IBM AT became a widely copied design in the booming home computer market of 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. Since 1997, the AT form factor has been largely supplanted by

ATX ATX (Advanced Technology Extended) is a motherboard and power supply configuration specification, patented by David Dent in 1995 at Intel, to improve on previous de facto standard, ''de facto'' standards like the AT (form factor), AT design. ...

Design

The original AT

, later known as "Full AT", is , 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 bay A drive bay is a standard-sized area for adding hardware to a computer. Most drive bays are fixed to the inside of a case, but some can be removed. Over the years since the introduction of the IBM PC, it and its compatibles have had many form f ...

s, making installation of new drives more difficult. (In IBM's original heavy-gauge steel case, the two " full-height drive bays overhang the front of the motherboard. More precisely, the left bay overhangs the motherboard, while the right bay is subdivided into two half-height bays and additionally extends downward toward the bottom of the chassis, allowing a second full-height fixed disk to be installed below a single half-height drive.) The power connectors for AT motherboards are two nearly identical 6-pin plugs and sockets. As designed by IBM, the connectors are mechanically keyed so that each can only be inserted in its correct position, but some clone manufacturers cut costs and used unkeyed (interchangeable) connectors. Unfortunately, the two power connectors it requires are not easily distinguishable, leading many people to damage their boards when they were improperly connected; when plugged in, the two black wires on each connector must be adjacent to each other, making a row of four consecutive black wires (out of the total 12). Technicians developed

mnemonic device A mnemonic device ( ), memory trick or memory device is any learning technique that aids information retention or retrieval in the human memory, often by associating the information with something that is easier to remember. It makes use of e ...

s to help assure proper installation, including "black wires together in the middle" and "red and red and you are dead".

Variants

In 1987, the Baby AT form factor was introduced, based on the motherboard found in the IBM PC/XT 286 (5162) and soon after all computer makers abandoned AT for the cheaper and smaller Baby AT form factor, using it for computers that spanned several generations, from those that used 286 processors to the P5

Pentium Pentium is a series of x86 architecture-compatible microprocessors produced by Intel from 1993 to 2023. The Pentium (original), original Pentium was Intel's fifth generation processor, succeeding the i486; Pentium was Intel's flagship proce ...

and a limited number of

Pentium II The Pentium II is a brand of sixth-generation Intel x86 microprocessors based on the P6 (microarchitecture), P6 microarchitecture, introduced on May 7, 1997. It combined the ''P6'' microarchitecture seen on the Pentium Pro with the MMX (instruc ...

systems. These motherboards have similar mounting hole positions and the same eight card slot locations as those with the AT form factor, but are wide and marginally shorter than full-size AT boards, with a maximum length of . However, Baby AT boards were mostly shorter than this, typically . The size and flexibility of this kind of motherboard were the key to success of this format. The development of bigger CPU coolers—and the fact that they blocked full-length PCI and ISA cards—spelled the end of Baby AT and was the main impetus for its successor

. While the AT standard is now considered to be mostly obsolete, some industrial computers still use it. In 1995,

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...

introduced

, a form factor which gradually replaced older Baby AT motherboards. During the late 1990s, a great majority of boards were either Baby AT or ATX. Many motherboard manufacturers favored Baby AT over ATX as many computer cases and power supplies in the industry were still designed for AT boards and not ATX boards. Also, the lack of an eighth slot on ATX motherboards kept it from being used in some servers. Later Baby AT boards supported both AT and ATX power connectors in addition to ATX features such as standby power (allowing for a low voltage power switch, as well as Wake-on-LAN/Wake-on-Modem Ring) and

USB Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical ...

by use of an ATX Form Card. After the industry shifted to ATX motherboard configurations, it became common to design cases and power supplies to support both Baby AT and ATX motherboards.

Power connector

The connector at the board is two Molex 15-48-0106 connectors. This mates with a Molex 90331.

References

External links

PC Power Supply LinksAT power supply connectors with pinoutsPC Magazine
{{Computer form factors At form factor Motherboard form factors