IBM Personal Computer AT
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| Also known as | PC/AT |
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| Manufacturer | IBM |
| Type | Personal Computer |
| Release date | August 14, 1984 |
| Introductory price | Approx. $6000 |
| Discontinued | April 2, 1987 |
| Units sold | 100,000+ |
| Operating system | PC DOS 3.0 and later, OS/2 1.x, PC/IX 1.1, IBM & SCO Xenix |
| CPU | Intel 80286 @ 6 and 8 MHz |
| Memory | 256 KB ~ 16 MB |
| Storage | 20 MB hard drive, 1.2 MB HD 135 mm (5.25") floppy |
| Input | Parallel, Serial |
| Predecessor | IBM Personal Computer XT |
| Successor | IBM Personal System/2 |
| Related articles | IBM Personal Computer |
The IBM Personal Computer AT, more commonly known as the IBM AT and also sometimes called the PC AT or PC/AT, was IBM's second-generation PC, designed around the 6 MHz Intel 80286 microprocessor and released in 1984 as System Unit 5170. The name AT stood for "Advanced Technology", and was chosen because the AT offered various technologies that were then new in personal computers; one such advancement was that the 80286 processor supported protected mode. IBM later released an 8 MHz version of the AT.
The IBM PC AT came with a 192-watt switching power supply. According to IBM's documentation, in order to function properly, the AT power supply needed a load of at least 7.0 amperes on the +5V line and a minimum of 2.5 amperes was on its +12V line. In practice, the AT power supply would randomly fail to start unless these minimum load requirements were met. Because the AT motherboard didn't provide much load on the +12V line, entry-level IBM AT models that didn't have a hard drive were shipped with a 5-ohm, 50-watt (maximum power) sandbar resistor connected on the +12V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (28.8 watts), getting fairly hot.
In addition to the unreliable hard disk drive, the high-density floppy disk drives turned out to be problematic. Some ATs came with one high-density (HD) disk drive and one double-density (DD) 360 kB drive. High-density floppy diskette media were compatible only with high-density drives. There was no way for the disk drive to detect what kind of floppy disk was inserted, and the only clue the user had was the disk label and an asterisk molded into the 360 kB disk drive faceplate. If the user accidentally used a high-density diskette in the 360 kB drive, it would sometimes work, for a while, but the high-coercivity oxide would take a very weak magnetization from the 360 kB write heads, so reading the diskette would be problematic.
A different problem occurred when using a double-density diskette in the 1.2 MB drive; the high-density drive's heads had a track width half that of the 360 kB drive, so they were incapable of fully erasing and overwriting tracks written by a 360 kB drive. Therefore, overwriting a DD disk that had been written to in a DD drive with an HD drive would result in a disk perfectly readable on an HD drive, but producing many read errors in a DD drive. Whereas a HD read head would only pick up the half track that drive had written, the wider DD read head would pick up the half-track written by the HD drive mixed with the unerased half-track remnant of the track written earlier by a DD drive. Thus, the DD drive would end up reading both new and old information together, causing it to "see" garbled data.
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