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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, IBM released a version of the AT running at 8 MHz.

AT features

  • 16-bit ISA bus – The AT motherboard had a 16-bit data bus and supported both 8-bit PC-style expansion cards as well as the new 16-bit AT expansion cards for the 80286
  • 15 IRQs and 7 DMA channels, expanded from 8 IRQs and 4 DMA channels for the PC. IRQ 8–15 are cascaded through IRQ 2, which leaves 15 active instead of 16. Similarly, DMA channel 4 is reserved for cascading 0–3 leaving 7 channels active.
  • 16 MiB maximum memory supported by the 24-bit address bus, compared to the PC's 640 kiB
  • battery backed Real-time clock on motherboard with 50 bytes CMOS memory available for power-off storage of BIOS parameters. (the basic PC had required either manual setting of its software clock using Time and Date commands, or the addition of an accessory expansion card with real-time clock, to avoid the default 01-01-80 file date)
  • 84-key AT keyboard layout – the "84th key" being <SysRq> i.e. System Request; numerical keypad now clearly separated from main key group; also added indicator LEDs for Caps lock/Scroll lock/Num lock. The AT keyboard uses the same 5-pin DIN connector as the PC keyboard, and is electrically compatible with it, but it generates different keyboard scan codes.
  • 1.2 MiB 5 1/4 inch floppy disk drive (15 sectors of 512 bytes, 80 tracks, 2 sides) stored over three times as much data as the 360 kiB PC floppy disk (9 sectors of 512 bytes, 40 tracks, 2 sides)
  • 20 MB hard disk drive was twice as fast (about 40 msec) as the PC XT's 10 MB drive, although the early drives manufactured by Computer Memories (CMI) had a 25–30% failure rate after one year. This was attributed partly due to failure to automatically retract the read/write heads when the computer was powered off, and partly due to a bug in DOS 3.0 FAT algorithm.
  • optional Enhanced Graphics Adapter with 16 display colors from a 64 color palette on a 640 x 350 pixel resolution screen
  • optional Professional Graphics Controller with 256 colors from a 4096 color palette and 640 x 480 resolution and accelerated 2D and 3D display functions for Computer Aided Design (CAD) applications
  • PC-DOS 3.0 was released to support the new AT features

PC/AT clones

IBM's efforts to trademark the name AT largely failed, and numerous clones appeared. "AT" became a standard term referring to any computer utilizing a 286 or faster processor. After the release of Intel's ATX motherboard, case, and power supply specifications in 1995, "AT" came to designate 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 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.

See also

IBM hardware | IBM PC compatibles

References

IBM PC AT | PC/AT | IBM PC/AT | IBM Personal Computer/AT | IBM PC AT

 

This article is licensed under the GNU Free Documentation License. It uses material from the "IBM Personal Computer/AT".

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