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 model number 5170. Because the AT used various technologies that were new at the time in personal computers, the name AT originally stood for Advanced Technology, as the Intel 80286 processor used in the AT supported Protected mode. IBM later released an 8 MHz version of the AT.
- AT-Bus: The AT motherboard had a 16-bit data bus and 24-bit address bus that was backward compatible with PC-style expansion cards (which were 8-bit data, 20-bit address).
- Fifteen IRQs and seven DMA channels, expanded from eight IRQs and four DMA channels for the PC. IRQs 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 seven channels active.
- Over 15 MB maximum memory (because of the 24-bit address bus of the 286), compared to the PC's 640 KB.
- 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
Date commands, or the addition of an accessory expansion card with real-time clock, to avoid the default
01-01-80 file date.)
- Eighty-four 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 MB 5-1/4 inch floppy disk drive (15 sectors of 512 bytes, 80 tracks, two sides) stored over three times as much data as the 360 KB PC floppy disk (nine sectors of 512 bytes, 40 tracks, two sides)
- A 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 the DOS 3.0 FAT algorithm.
- An optional Enhanced Graphics Adapter with 16 display colors from a 64 color palette on a 640 × 350 pixel resolution screen
- An optional Professional Graphics Controller with 256 colors from a 4096 color palette, a 640 × 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.
- The AT was equipped with a lock and key that could be used to prevent access to the computer.
In addition to the unreliable hard disk drive, there were problems with the floppy disk drives:
- The high-density floppy disk drives turned out to be highly problematic. Some AT's came with one high-density disk drive and one regular 360 kB drive. 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 a subtle asterisk molded into the 360kB disk drive faceplate. If you accidentally used a high-density diskette in the 360 kB drive, it would sometimes work, for a while, but the high-remanence oxide would take a very weak magnetization form the 360 kB write heads, so reading the diskette would be problematic. The same problem occurred when using a low-density diskette in the 1.2 MB drive; things would appear to work for a while, but often the diskette would go steadily downhill after a few read/write cycles. In addition, the HD drive heads produced finer tracks than the DD drive heads—so overwriting a DD disk that had been used in a DD drive with a HD drive would result in a disk perfectly readable on a HD drive, but producing many read errors in a DD drive: The HD write head would have renewed only a small part of the originally available broad DD track, and whereas a HD read head would only see this fine and small part, a DD read head would end up reading both new and old information together.
The combination of the faster clock rate, fewer clock cycles per instruction, and the 16-bit bus led to a computer that was in the marketing sense too fast. IBM was protective of their lucrative mainframe and minicomputer businesses and consequently ran the original PC/AT (139 version) at a very conservative 6 MHz with one wait state. They also used a three to one interleave on the hard disk, even though the controller supported two to one. Many customers replaced the 12 MHz crystal (which ran the processor at 6 MHz) with a 16 MHz one, so IBM introduced the PC/AT 239 which would not boot the computer at any speed faster than 6 MHz, by adding a speed loop in the ROM. This also introduced the Baby AT motherboard form factor. The final PC/AT, the 339, ran the processor at 8 MHz with one wait state, and was built as IBM's flagship microcomputer until the 1987 introduction of the PS/2 line.
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
motherboard, case, and power supply specifications in 1995, "AT" came to designate motherboards whose size and screw positions approximated those of IBM's original AT form factor 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.
- IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
- PC AT entry at old-computers.com