The ATX form factor was the first of a dramatic evolution in motherboard form factors. ATX is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with a modified power supply location and connector.

The most important thing to know initially about the ATX form factor is that it is physically incompatible with either the previous Baby-AT or LPX design. In other words, a different case and power supply are required to match the ATX motherboard. These case and power supply designs have become common and are found in most new systems.

Intel initially released the official ATX specification in July 1995. It was written as an open specification for the industry. ATX boards didn't hit the market in force until mid-1996, when they rapidly began replacing Baby-AT boards in new systems.

The ATX specification was updated to version 2.01 in February 1997, and has had several minor revisions since. The latest revision is ATX version 2.03 (with Engineering Change Revision P1), released in May 2000. Intel has published detailed specifications so other manufacturers can use the ATX design in their systems.

The current specifications for ATX and other current motherboard types are available online from the Desktop Form Factors site. Currently, ATX is the most popular motherboard form factor for new systems, and it is the one I recommend most people get in their systems today.

An ATX system will be upgradeable for many years to come, exactly like Baby-AT was in the past. ATX improved on the Baby-AT and LPX motherboard designs in several major areas:

• Built-in double high external I/O connector panel. The rear portion of the motherboard includes a stacked I/O connector area that is 6 1/4'' wide by 1 3/4'' tall. This enables external connectors to be located directly on the board and negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

• Single main keyed internal power supply connector. This is a boon for the average end user who always had to worry about interchanging the Baby-AT power supply connectors and subsequently blowing the motherboard. The ATX specification includes a keyed and shrouded main power connector that is easy to plug in and can't be installed incorrectly.

This connector also features pins for supplying 3.3V to the motherboard, so ATX motherboards do not require built-in voltage regulators that are susceptible to failure. The ATX specification was extended to include two additional optional keyed power connectors called the Auxiliary Power connector (3.3V and 5V) and the ATX12V connector for systems that require more power than the original specification would allow.

• Relocated CPU and memory. The CPU and memory modules are relocated so they can't interfere with any bus expansion cards and can easily be accessed for upgrade without removing any of the installed bus adapters. The CPU and memory are relocated next to the power supply, which is where the primary system fan is located.

The improved airflow concentrated over the processor, in the case of some older processors, eliminates the need for extra-cost CPU cooling fans (lower power configurations only). There is room for a CPU and a heatsink and fan combination of up to 2.8'' in height, as well as more than adequate side clearance provided in that area.

• Relocated internal I/O connectors. The internal I/O connectors for the floppy and hard disk drives are relocated to be near the drive bays and out from under the expansion board slot and drive bay areas. Therefore, internal cables to the drives can be much shorter, and accessing the connectors does not require card or drive removal.

• Improved cooling. The CPU and main memory are designed and positioned to improve overall system cooling. This can decrease—but not necessarily eliminate—the need for separate case or CPU cooling fans. Most higher-speed systems still need additional cooling fans for the CPU and chassis.

Note that the ATX specification originally specified that the ATX power supply fan blows into the system chassis instead of outward. This reverse flow, or positive pressure design, pressurizes the case and minimizes dust and dirt intrusion. More recently, the ATX specification was revised to allow the more normal standard flow, which negatively pressurizes the case by having the fan blow outward.

Because the specification technically allows either type of airflow, and because some overall cooling efficiency is lost with the reverse flow design, most power supply manufacturers provide ATX power supplies with fans that exhaust air from the system, otherwise called a negative pressure design.

• Lower cost to manufacture. The ATX specification eliminates the need for the rat's nest of cables to external port connectors found on Baby-AT motherboards, additional CPU or chassis cooling fans, or onboard 3.3V voltage regulators.

Instead, ATX allows for shorter internal drive cables and no cables for standard external serial or parallel ports. These all conspire to greatly reduce the cost of the motherboard and the cost of a complete system—including the case and power supply.

Figure below shows the ATX system layout and chassis features, as you would see them looking in with the lid off on a desktop, or sideways in a tower with the side panel removed.

Notice how virtually the entire motherboard is clear of the drive bays and how the devices such as CPU, memory, and internal drive connectors are easy to access and do not interfere with the bus slots. Also notice how the processor is positioned near the power supply.

The ATX motherboard shape is basically a Baby-AT design rotated sideways 90°. The expansion slots are now parallel to the shorter side dimension and do not interfere with the CPU, memory, or I/O connector sockets. There are actually two basic sizes of standard ATX boards.

In addition to a full-size ATX layout, Intel also specified a mini-ATX design, which is a fully compatible subset of ATX that fits into the same case:

• A full-size ATX board is 12'' wide x 9.6'' deep (305mmx244mm).

• The mini-ATX board is 11.2'' x 8.2'' (284mmx208mm).

Mini-ATX is not an official standard; instead it is simply referenced as a slightly smaller version of ATX. In fact, all references to mini-ATX were removed from the ATX 2.1 and later specifications. Two smaller official versions of ATX exist, called micro-ATX and flex-ATX.