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AT vs. ATX
How do I tell what type of case, motherboard or power supply I have

AT and ATX are formfactors (standards) that Motherboards, Power Supplies and Case manufacturers can conform to, to make sure they are interchangeable with other parts. These formfactors are important to the end user also, so they can make sure to get replacement parts that will work and, more importantly, fit with what they already have.

It is important to note that many distributors (name brand) computers may conform to one or more of these standards but not to the rest. So it is important, if you are trying to find a replacement for a part, that if possible you take it with you to the store, or match it with a friends part to make sure it is standard. If your part is not standard, then you will probably have to go back to the distributor of your PC for a replacement.

Since the most common, and easiest thing to replace in a computer is the power supply, we will start there. The power supply is located in the back of the case, and in the top (for towers) or to the right (looking from the front in desktops). It is the box that the power cord plugs into from the outside. On the inside, it is where the bundle of colored wires come out of. On the power supply should be a label giving voltages, ratings and a warning, also there should be something about 'AT' or 'ATX'. Since most motherboards use standard power connectors, the label may just say which motherboard power connector it has, so don't rely on this too much. The power supply is mounted by four screws from the back of the case, and may be held in place, internally, by a bracket. The connectors on the end of the wires will be of three different types. First, the 3 1/2" floppy drive power connector is 1/2" wide, and there are usually two of these. The second type of connector is for other drives (hard drives, CD-ROM, etc.) and is 3/4" wide. The wires going to the motherboard are will define whether it is 'AT' or 'ATX'. 'AT' types will have a connector from the power supply that is for a single row of pins and will be divided in two. 'ATX' types will have a block connector for two rows of pins that is the same length as 'AT'. 'AT' power supplies will also have some wires that go to the front of the case and connect to the power switch. 'ATX' has the power switch plug into to the motherboard. Note: some motherboards that appear to be ATX will also have a second connectors from the power supply to the motherboard that looks like the AT connector but 1" long. This makes your power supply and motherboard proprietary, although some people have been able to replace their power supply and have no problems with nothing plugged into the extra power connector on the motherboard.

Next, is the motherboard. Motherboard may be the more difficult of the three to replace if you have a brand name PC. Large companies, in an effort to lower their prices, have opted to leave some features out or integrate them into the motherboard and make them proprietary. These motherboards may look to follow one of the formfactors but a standard replacement will probably not fit into the cases made for them. One of the easier ways to identify what formfactor your motherboard is, is to look at the back of your computer. Is the place you plug your keyboard into a hole in the case by itself and about 1/2" across? Then you have an 'AT' motherboard. Is the keyboard port 3/8" across? Then you have a PS/2 keyboard connector which is part of the ATX formfactor. Are the rest of the connectors arranged the same as in the picture? Then you have an ATX formfactor on the outside of your motherboard.