AT and baby AT form factor

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The AT and baby AT form factors were the earlier form of motherboards which were quite advanced according to the standards of time. As the name suggests, baby AT is an AT motherboard which is smaller in size.

AT and baby AT form factor

AT and baby AT form factor

A baby AT motherboard measures 8.5″ by 13″. The baby AT form factor is being replaced by ATX motherboards. The PC-AT motherboard used to measure 12″ by 13″.

There is a baby AT motherboard that is 8.5″ wide that was introduced because the full-sized AT motherboard, at 12″ wide, was too wide to fit into the newer, more common mini-towers and desktop cases that are being manufactured. In general, there are hardly any full-sized AT motherboards out there today, and fewer cases are available to hold them. A full tower case will be needed in order to mount an AT motherboard in a full tower case.


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Baby AT motherboards

AT and baby AT form factor

Baby AT motherboards have a reduced width, which means that they overlap less with drive bays, although there may still be overlaps at the front. Recently, the size of the baby AT board has been reduced even further to 11″ or 10″ long, which can lead to mounting problems when the motherboard is mounted in the case.

In addition to their shape and the one full-size keyboard connector soldered on the board, AT and baby AT motherboards can be identified. A serial or parallel port connector is usually connected to the motherboard via cables, which connect the pin connectors to the case.

As you may know, the processor socket on AT and baby AT motherboards is usually at the front of the motherboard, and the memory socket is usually at the rear. They were designed to accommodate long expansion cards and drives. The older 386 and 486 processors had relatively small CPUs and 30-pin memory, so clearance over them was not a problem. As SIMM and DIMM memory systems have increasingly taken over, as well as Pentium and faster processors that have large heat sinks and fans that have increased their cooling requirements, the combination can often result in two or more expansion slots being blocked and/or a drive bay being obstructed. 

Several motherboard manufacturers are moving the memory sockets closer to the keyboard connector as a means of addressing this problem, but the size of the processor still remains a challenge. The ATX form factor and case attempts to address this problem partially by repositioning the memory sockets closer to the keyboard connector.

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AT and baby AT form factor

AT motherboards are 13.8 inches by 12 inches (351 by 305 millimeters), so they will not fit in cases with “mini desktops” or “minitowers”. As a result of the board’s size, it takes up more space behind the drive bays, which makes it more difficult to install new drives. Two full-height drive bays overhang the motherboard in IBM’s original heavy-gauge steel case. It is more specifically designed to allow two full-height fixed disks to be installed beneath a single half-height drive. The left bay overhangs the motherboard, while the right bay is divided into two half-height bays that extend toward the chassis’ bottom.)

It is possible to plug and socket AT motherboards with two almost identical 6-pin power connectors. Some clone manufacturers cut costs and used unkeyed (interchangeable) connectors instead of IBM’s mechanically keyed connectors that can only be inserted in the correct position. Many people damaged their boards when they improperly connected the two power connectors it requires, as they are not easily distinguishable. A row of four consecutive black wires (out of a total of 12) must form when both black wires on each connector are adjacent to each other when plugged in. Several mnemonic devices were developed to ensure proper installation, including “black wires together in the middle” and “red and red or you’ll die”. Read more on  Best motherboards for nzxt h510

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Powered by USB, PS/2 mouse, and IR jacks, this ATX Form Card is used by later Baby-AT motherboards.

AT and baby AT form factor

Baby AT was introduced in 1987, based on the motherboard found in IBM PC/XT 286 (5162) and shortly thereafter, all computer makers began to abandon the AT form factor for the Baby AT form factor, which was cheaper and smaller. During this time, it was used in a wide range of computer systems, ranging from 286 processors to P5 Pentiums and some Pentium IIs, covering multiple generations. A typical motherboard of this type has eight slot locations and the same positioning of mounting holes as a motherboard in the AT form factor.

However, the motherboard is 8.5 in wide (216 mm) and is marginally shorter than full-size AT boards, with a maximum length of 13 in (330 mm) compared to full-size AT boards. Baby AT motherboards were generally smaller than this, typically 9 to 10 in (229 to 254 mm) long. This type of motherboard was very popular due to its flexibility and size. While the AT standard is now mostly considered to be obsolete, some industrial computers still use it due to the development of bigger CPU coolers and the fact that these coolers block full-length PCI and ISA cards. When the AT standard was designed, it was basically meant to replace the AT standard, but some manufacturers still implement it in their products.

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Enter the ATX


During the late 1990s, most motherboards were either Baby AT or ATX, as Intel introduced the ATX form factor in 1995 to gradually replace older Baby AT motherboards. There were still a lot of computer cases and power supplies that were made for AT boards, so many motherboard manufacturers preferred Baby AT over ATX. In addition, some servers weren’t able to use it since ATX motherboards didn’t have an eighth slot.

 The Baby AT board was later updated to support both AT and ATX power connectors, as well as ATX features such as standby power (a low voltage switch was implemented, along with a Wake-on-LAN and Wake-on-Modem Ring, and USB with the use of an ATX Form Card) to enable these features. When the industry started switching to ATX motherboard configurations, it became more common for power supplies and cases to be designed to be compatible with both Baby AT and ATX motherboards.

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