IBM Personal Computer/AT
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The IBM Personal Computer/AT (IBM 5170), 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 Intel 80286 microprocessor running at 6 MHz and released in 1984. Because the AT used various technologies that were rare at the time in personal computers, the name AT originally stood for Advanced Technology, and indeed, the Intel 80286 processor used in the AT supported Protected mode. Later, there was a second version of the AT running at 8 MHz.
IBM's efforts to trademark the name AT largely failed, and numerous clones appeared. "AT" eventually became a standard term referring either to any computer utilizing a 286 processor or better, or, especially after the release of Intel's competing ATX specification, for motherboards whose size and screw positions approximated those of IBM's original standard, power supplies that could plug into them, and cases that could house them.
The AT architecture was an ad hoc standard, and while the power supplies and motherboards that fit in one AT case usually fit another, the specifications were not universal and there were sometimes physical incompatibilities. AT compatible features include the location of the keyboard and expansion slot connectors on the motherboard and corresponding openings on the case, and the physical and electrical characteristics of the motherboard power connector and the speaker connector. An AT-compatible power supply has a cooling fan and four mounting holes in specific locations and usually a pushbutton power switch at the end of an approximately half-meter long power cable, although the original PC/AT used a toggle switch mounted directly to the power supply. Disk drive size, connectors and mounting points are not strictly part of the AT standard; the same drive types are used in AT, PS/2 and ATX compatible computers.
A so-called “AT keyboard” refers to a computer keyboard that is physically and electrically compatible with the original PC/AT. Like the original PC keyboard these use a full-size 5-pin DIN connector, but the keypress encoding is incompatible. In addition the computer can control the indicator lights for “CAPS Lock”, “NUM Lock” and “Scroll Lock” on an AT keyboard, but the original PC keyboard did not have these indicators.
A PS/2 keyboard is electrically compatible with an AT keyboard, but uses a 6-pin mini-DIN connector. An AT keyboard can be plugged into a PS/2 connector via a converter, and vice versa.