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PC/XT Form Factor
The first PC was of course the IBM PC. Its power supply, and that of its hard-drive-equipped successor, the IBM PC/XT, used the same original form factor. These systems were all desktop units, with the power supply tucked into the rear of the case on the right-hand side, and controlled via an up/down toggle switch. While the PC/XT power supply began as an IBM design, IBM's key decision to keep the PC architecture open allowed "clone" manufacturers to make similar PC boxes and use the same size and shape of power supply for interoperability. In this manner, the first PC form factor "standard" was born.
PC/XT units were sold as desktop boxes only. Equipped with only one or two 5.25" (low-density) floppy disk drives for storage, and having limited expansion possibilities, the original PC came with a very low-powered supply by today's standards: 63.5 W. The XT added the first PC hard disk drive and an appropriately doubled power supply rating: 130 W (still relatively small by today's standards.) These IBM power supplies were physically large for their output--not surprising, since they used much older components and were designed before some power supply functions were combined into integrated circuits. They were also very well-made, another reason for their somewhat large size. They of course are used in PC/XT form factor system cases and with PC/XT motherboards. The PC/XT was the first form factor to use the well-known pair of six-conductor motherboard connectors that were used through the Baby AT and LPX form factors, and the four-conductor disk drive connectors that are still used to this day.
These units are of course not only obsolete today but heading rapidly into "antique" status. You will still find them in use however, usually as dumb terminals for larger minicomputers, or for controlling industrial equipment. This is as much a testament to the quality of these first PC power supplies as anything.
Next: AT Form Factor