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What's the Difference Between AT and ATX Form Factors?

What is the big difference between AT and ATX formats? The terms basically describe the shape and size of the motherboards, as well as the layout of the components on the board. The ATX is the most common today, rarely will you see the older AT format in the market. I'll provide an quick overview of each for those curious. The case and power supply must then match the type of motherboard you have chosen.

The AT Form Factor

Within the AT form, we have regular AT and Baby AT. They basically differ in size. An AT board is about 12" wide which means it can't fit in many of today's cases. AT boards generally are the older boards, 386 or earlier. Working inside the case was a lot more trouble with these because the size of the motherboard overlapped drive bays and such.

Baby AT is the form used by many boards and cases today, although it is phasing out to ATX. Many Socket 7 motherboards and a few Pentium II boards use this form factor. A Baby AT board is roughly 8.5" wide and 13" long. The size varies a little from board to board. This reduced size makes it easier to work inside the case simply because there is more room. There are three rows of mounting holes to hold the board in the case.

AT form boards share common traits. They all have serial and parallel ports attached to the case in an expansion slot and connected to the board through cables. They also have a single keyboard connector soldered onto the board at the back of the board. The processor is still at the front of the board and can sometimes get in the way of expansion cards. The SIMM slots are in different places, although they are almost always at the top of the board.

The industry is making moves away from the AT form factor, simply because ATX, discussed below, is much more advanced. There are some annoyances with the AT design. One is due to the layout. Since all ports are attached to the case and then connected to the motherboard via a cable, the board must have connectors for all of these: COM 1, COM 2, printer port, USB, PS/2 mouse, etc. Often these connectors are directly next to the IDE channel connectors and floppy drive connector. This leads to a severe cramping problem and makes working inside the computer more difficult.

Secondly, the AT design is not conducive to efficient cooling of the system. Air is not blown over the areas that need it, namely the CPU. Also, the air flow draws in dust. Over time, the AT power supply will get dusty and the inside of the system will be coated with a layer of dust. For this reason, it is recommended you regularly remove the case and blow off the interior of the case.

The ATX Form Factor

Many consider the ATX design a vast improvement over the standard AT design. It fixes the annoyances of the AT form. Since the AT form was so old, as new demands were placed on it from new technology, little problems began to show up. These problems are taken into account with the ATX format which Intel released in 1995. It was slow to catch on due to the long-time acceptance of the AT board, but slowly it gained popularity. Almost motherboards today are ATX. The AT form factor is rarely found today.

Some advanced features of the ATX format:

  • Integrated I/O Connectors
    While the AT uses headers on the board that are attached to the actual ports on the back of the case, the ATX board has the actual ports built right onto the board. This makes installation easier and enhances reliability. This includes an integrated PS/2 mouse connector.
  • Reduced Overlap Between Board and Drives
    The ATX board looks like it is rotated 90 degrees so that it does not overlap the drive bays in the front. This way, one can reach the entire board instead of having to reach around a drive, or even remove the drive, in order to reach certain areas of the motherboard. This also reduces heat.
  • Reduced Processor Interference with Cards
    The processor is moved from the front of the board near the slots to the back, top of the board, near the power supply. This means that a user can install full-length expansion cards in the slot without having to worry about hitting the CPU or heat sink.
  • User-Friendly Power Connector
    ATX uses one 20-pin connector to attach to the motherboard. The ATX connector is also keyed so that it will only go in the correct way. This is easier than the two separate connectors of the AT form which look almost the same. It also gets rid of the problem of frying the board due to misplacement of the connectors on the motherboard.
  • Better Cooling Conditions
    The ATX power supply blows air into the case instead of out. This means that air blows out all the holes in the case and thus keeps dust out.
  • 3.3 Volt Power
    The ATX motherboard is designed to accept 3.3 volt power directly from the power supply. Since almost all modern processors operate at 3.3 volts, this removes the need for a voltage regulator on the motherboard to reduce the voltage from 5V to 3.3V. It must be noted, though, that many processors don't use this voltage, and therefore must use a voltage regulator anyway.
  • Automated Controls
    The ATX power supply is capable of being controlled through software and other means. This is because it always has a slight voltage going through it. This gives the computer the ability to turn itself on at specified times and perform some task given to it by the software. Some ATX motherboards have the option of turning the system on by pressing the space bar on your keyboard or being woken up by a command sent down through the LAN. Lastly, the shut down procedure is automated using ATX. When you choose "Shut Down", the computer will perform all shut down tasks, then turn itself off.