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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 8: Motherboards
      9  Form Factors

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ATX and Mini ATX

ATX represents the “modern era” of motherboards. The ATX “dethroned” the AT and Baby AT form factors and is found commonly in most computers today. The ATX originally housed the Intel Pentium Pro and Pentium II processors and of course has moved on to the current Pentium IV.

Let’s start with a summary of the ATX form factor. It measures 12 inches (30,48 cm) wide and 9.6 inches (24,38 cm) deep while the smaller scale version called the Mini ATX measures 11.2 inches (28,45 cm) wide by 8.2 inches (20,83 cm) deep. The major improvement of the ATX over prior designs was not just one of size and shape but it specified completely different case and power supply requirements. Since this is the “modern” form factor, the ATX will be recognizable by all technicians who commonly work on personal computers. Even without opening up the case, you will see the familiar PS/2 mouse connector and the keyboard connector as well as four USB connectors. Of course there will also be a serial connector and a parallel (printer) connector as well as a monitor connector and an on board audio card. The ATX also has built in “soft” power support instead of the toggle style power switch used by the AT form factor.

ATX85 was introduced in 1995 by Intel however; it did not really start catching on until the Pentium Pro. Contrary to statements on the web (and possibly the correct choice, if not a technically correct answer on the A+ test), not all Pentium Pro motherboards were ATX.

Here are the specific differences between an AT and ATX boards:

  • Integrated I/O Connectors: ATX boards have the actual ports built right onto the board. This makes installation easier and improves reliability.

    An integrated PS/2 mouse connection and the keyboard connection were also reduced in size from the 5 pin DIN to a PS/2. Later on, these became color coded with blue for the keyboard and green for the mouse. If you happen to be the roughly 10% of males
    86 who have the most common type of visual color blindness <deuteranopic87> (red/green) burn “green mouse” into your brain. This factoid may come in handy later.

  • Reduced Overlap between Board and Drives: The ATX board is rotated 90 degrees so that it does and makes it easier to work on the system board without working around or uninstalling a drive as well as reducing heat.

  • Reduced Processor Interference with Cards: As the CPU has increased heat output (a 1/8” square CPU can now equal the heat generated of a 100-Watt light bulb) heat sinks have gotten larger. Therefore, Intel has moved the CPU from the front of the board near the slots to the back and top of the board, near the power supply. This made installing a full-length card less of an issue.

  • User-Friendly Power Connector: ATX uses one 20-pin (2 by 10) keyed connector to attach to the motherboard. This is easier than the two connectors that look almost the same in the AT design. No longer do you have to remember to adjoin P8 and P9 connectors so the black cables are next to each other when installing AT power.

  • Better Cooling Conditions: Placing the CPU closer to the power supply provides for better cooling.

  • 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.

  • Soft Switch: The AT power supply uses a mechanical switch to supply power (or not). In the ATX, the power is always on. It may be in standby mode (making the computer appear 'off'). The beauty of a 'soft switch' as found in the ATX is, this makes it possible to return the system to a full power state, sometimes called “Wake on” with Wake On Modem88, Wake on Keyboard/Mouse89 or WOL (Wake On LAN) with a magic packet90. This last feature is really a money saver. You can broadcast to an entire group of computers on a LAN and push an application update out during off hours.








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CertiGuide to A+ (A+ 4 Real) ( on
Version 1.0 - Version Date: March 29, 2005

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