IBM Personal Computer AT

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The IBM Personal Computer AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80286 microprocessor.

IBM PC/AT (System Unit 5170)
Also known asIBM AT, PC/AT
TypePersonal Computer
Release dateAugust 14, 1984; 39 years ago (1984-08-14)[1]
Introductory priceApprox. US$6,000 (equivalent to $18,400 in 2023)
DiscontinuedApril 2, 1987[2]
Units sold100,000+
Operating system
CPUIntel 80286 @ 6 and 8 MHz
Memory256 KB – 512 KB onboard + 3.5 MB with additional memory cards
Storage20 MB hard drive
1.2 MB HD 5.25" (135 mm) floppy drive
GraphicsCGA, EGA
InputParallel, serial
Power120/240 VAC
PredecessorIBM Personal Computer XT
SuccessorIBM PS/2 Model 30 286
RelatedList of IBM Personal Computer models



IBM did not specify an expanded form of AT on the machine, press releases, brochures or documentation, but some sources[3] expand the term as Advanced Technology, including at least one internal IBM document.[4]



IBM's 1984 introduction of the AT was seen as an unusual move for the company, which typically waited for competitors to release new products before producing its own models. At $4,000–6,000, it was only slightly more expensive than considerably slower IBM models. The announcement surprised rival executives, who admitted that matching IBM's prices would be difficult. No major competitor showed a comparable computer at COMDEX Las Vegas that year.[3]

IBM 5170 with Floor Standing Enclosure[5] and compatible non-IBM display



The AT is IBM PC compatible, with the most significant difference being a move to the 80286 processor from the 8088 processor of prior models. Like the IBM PC, the AT supported an optional math co-processor chip, the Intel 80287, for faster execution of floating point operations.

In addition, it introduces the AT bus, later known as the ISA bus, a 16-bit bus with backward compatibility with 8-bit PC-compatible expansion cards. The bus also offered fifteen IRQs and seven DMA channels, expanded from eight IRQs and four DMA channels for the PC, achieved by adding another 8259A IRQ controller and another 8237A DMA controller.[6][7] Some IRQ and DMA channels are used by the motherboard and not exposed on the expansion bus. Both dual IRQ and DMA chipsets are cascading which shares the primary pair. In addition to these chipsets, Intel 82284 Clock Driver and Ready Interface and Intel 82288 Bus Controller are to support the microprocessor.

The 24-bit address bus of the 286 expands RAM capacity to 16 MB.

PC DOS 3.0 was included with support for the new AT features, including preliminary kernel support for networking (which was fully supported in a later version 3.x release).

The motherboard includes a battery-backed real-time clock (RTC) using the Motorola MC146818.[8][9] This was an improvement from the PC, which required setting the clock manually or installing an RTC expansion card. The RTC also included a 1024 Hz timer (on IRQ 8), a much finer resolution than the 18 Hz timer on the PC.[10]

In addition to keeping the time, the RTC includes 50 bytes of CMOS memory which is used to store software-adjustable BIOS parameters. A disk-based BIOS setup program which saved to this memory took the place of the DIP switches used to set system settings on PCs. Most AT clones have the setup program in ROM rather than on disk.



The standard floppy drive was upgraded to a 1.2 MB 5+14 inch floppy disk drive (15 sectors of 512 bytes, 80 tracks, two sides), which stored over three times as much data as the 360 KB PC floppy disk, but had compatibility problems with 360k disks (see Problems below). 3+12 inch floppy drives became available in later ATs.[citation needed]

A 20 MB hard disk drive was included as standard. Early drives were manufactured by Computer Memories and were found to be very unreliable.[11]



The AT included the AT keyboard, initially a new 84-key layout (the 84th key being SysRq). The numerical keypad was now clearly separated from the main key group, and indicator LEDs were added for Caps Lock, Scroll Lock and Num Lock. The AT keyboard uses the same 5-pin DIN connector as the PC keyboard, but a different, bidirectional electrical interface with different keyboard scan codes. The bidirectional interface allows the computer to set the LED indicators on the keyboard, reset the keyboard, set the typematic rate, and other features. Later ATs included 101-key keyboards, e.g. the Model M keyboard.

The AT is also equipped with a physical lock that prevents access to the computer by disabling the keyboard and holding the system unit's cover in place.

ATs could be equipped with CGA, MDA, EGA, or PGA video cards.

The 8250 UART from the PC was upgraded to the 16450, but since both chips had single-byte buffers, high-speed serial communication was problematic as with the XT.[12]



Power supply


The IBM PC AT came with a 192-watt switching power supply, significantly higher than the 130-watt XT 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 +5 V line and a minimum of 2.5 amperes on its +12 V line. The power supply would fail to start unless these minimum load requirements were met, but the AT motherboard did not provide much load on the +12 V line. To solve this problem, entry-level IBM AT models that did not have a hard drive were shipped with a 5-ohm, 50-watt resistor connected on the +12 V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (dissipating 28.8 watts), getting fairly hot.[24]



In addition to the unreliable hard disk drive,[25] 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 drives were not distinguished except by 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.

Conversely, 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. Overwriting a DD disk that had been written in a DD drive with an HD drive would result in a disk that read on an HD drive, but produced 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.



Due to[citation needed] a US antitrust consent decree with IBM, the PC AT architecture was functionally an open design, and IBM's efforts to trademark the AT name largely failed. Many 286-based PCs were modeled after it and marketed as AT-compatible. The label also became a standard term in reference to PCs that used the same type of power supply, case, and motherboard layout as the 5170. AT-class became a term describing any machine which supported the same BIOS functions, 80286 or greater processor, 16-bit expansion slots, keyboard interface, 1.2 MB 5+14 inch floppy disk drives and other defining technical features of the IBM PC AT.

In the United States, popular brands of AT clones included the Tandy 3000, Compaq Deskpro 286, HP Vectra,[26] Zenith Z-286,[27] Epson Equity Models II+[28] and III,[29] and Commodore PC-30 and PC-40. In Europe, on the other hand, most AT-clones sold were more or less anonymous.

The AT bus became the de facto ISA (Industry Standard Architecture), while PC XT slots were retroactively named 8-bit ISA. The disk interface was standardized as ATA which evolved and was later renamed PATA (parallel ATA). Further, the same interface was originally named IDE after the fact that the drive controller was on the drive (Integrated Drive Electronics) and not on the interface card. The name IDE stuck and is more commonly known, but ATA refers to the interface specifically.



As of January 1985 AT sales were so strong that IBM and its suppliers could not keep up with demand.[30]

Creative Computing chose the AT as the best desktop computer when "price is no object" for 1984, describing it as "an innovative, state-of-the-art computer that has the competition gasping for breath".[31] An industry analyst wrote in Computerworld in 1985 that the AT's power was evidence of IBM's belief that personal computers were more important for the company than minicomputers.[32]



On April 2, 1987, IBM announced the Personal System/2 (PS/2) line, which they marketed as the second-generation of IBM PC. The company promised to continue manufacturing certain models of the first-generation PC, including the AT, for the coming months.[33] In June 1987, they announced the full withdrawal of the PC/XT and the imminent discontinuation of the PC/AT. The last units of PC/AT (model 339) rolled off the assembly line in July.[15] While the PC/XT received a directly compatible replacement in the form of the PS/2 Model 30, the AT did not. Users either had to forgo all their 16-bit ISA expansion cards and switch to the proprietary Micro Channel architecture, or settle for a clone if they wanted to upgrade their machine while keeping their expansions. Eventually, in September 1988, IBM announced the PS/2 Model 30 286, which featured an Intel 80286 processor and 16-bit ISA expansion slots,[34][35] serving as the direct replacement for the AT for customers who wanted to buy a true IBM system.[36]


Timeline of the IBM Personal Computer
IBM ThinkCentreIBM NetVistaIBM Palm Top PC 110IBM PC SeriesIBM AptivaIBM PS/ValuePointThinkPadEduQuestIBM PS/noteAmbra Computer CorporationIBM PCradioIBM PS/1IBM Industrial SystemIBM PS/55IBM PS/2IBM Personal Computer XT 286IBM PC ConvertibleIBM JXIBM Personal Computer AT/370IBM Personal Computer ATIBM Industrial ComputerIBM PCjrIBM Portable Personal ComputerIBM Personal Computer XT/370IBM 3270 PCIBM Personal Computer XTIBM 5550IBM Personal ComputerIBM System/23 DatamasterIBM 5120IBM 5110IBM 5100
Asterisk (*) denotes a model released in Japan only

See also



  1. ^ Somerson, Paul (1984-11-13). "AT the Party". PC Magazine. p. 123. Retrieved 2014-07-05.
  2. ^ "IBM PC AT at Vintage Computer". Archived from the original on 2020-08-22. Retrieved 2010-10-20.
  3. ^ a b Sanger, David E. (1984-11-19). "I.B.M. Entry Unchallenged at Show". The New York Times. ISSN 0362-4331. Retrieved 2017-07-03.
  4. ^ IBM Personal System 2 and IBM Personal Computer Publication Reference Jan89 (PDF). 1989. p. 66.
  5. ^ "IBM PC-AT - UvA Computer Museum catalogue". University of Amsterdam. Retrieved 2023-07-15.
  6. ^ Derfler, Frank J. Jr. (1989-12-26). "Tutor". PC Magazine. Ziff Davis. p. 53. ISSN 0888-8507.
  7. ^ Mathivanan, N. (2007). PC-Based Instrumentation: Concepts and Practice. PHI Learning. pp. 227–229. ISBN 978-81-203-3076-4.
  8. ^ Brendan Horan (2013-03-26). Practical Raspberry Pi. Apress. p. 146. ISBN 978-1-4302-4972-6.
  9. ^ Shanley, Tom; Anderson, Don (1995). Swindle, John (ed.). ISA System Architecture (3 ed.). Mindshare, Inc. / Addison-Wesley Publishing Company. pp. 441–444. ISBN 0-201-40996-8.
  10. ^ Howard Austerlitz (2002). Data Acquisition Techniques Using PCs. Academic Press. pp. 90–91. ISBN 978-0-08-053025-3.
  11. ^ Dickinson, John (1985-06-25). "The AT's Slipped Disk". PC Magazine. p. 55. Retrieved 2013-10-28.
  12. ^ Nickalls, Richard W. D.; Ramasubramanian, R. (1995). Interfacing the IBM-PC to Medical Equipment: The Art of Serial Communication. Cambridge University Press. p. 25. ISBN 978-0-521-46280-8.
  13. ^ a b c d e f g h i j k l m IBM Personal System/2 and IBM Personal Computer Product Reference (PDF) (4.0 ed.). International Business Machines Corporation. 1987-04-06 – via Bitsavers.
  14. ^ a b Whitmore, Sam (1984-08-14). "IBM's New Generation: The PC AT: AT Hikes CPU Speed Sixfold, Memory Fivefold". PC Week. 1 (32). Ziff-Davis: 1 – via Gale. The IBM PC AT Base Model 68, with one 1.2-megabyte floppy-disk drive, will sell for $3,995, according to a source close to IBM. Model 99, with a 1.2-megabyte floppy-disk drive and a 20-megabyte hard disk, will cost $5,795. Both models will use a 6-MHz 80286 microprocessor.
  15. ^ a b c d e f g h i Abruzzese, George (1987-06-29). "IBM Plans to Terminate Remainder of PC Line". InfoWorld. 9 (26). IDG Publications: 1, 69 – via Google Books.
  16. ^ Kuzbyt, Bob (1985-10-07). "IBM introduces 30-MB 5.25-inch Winchester disk drive for AT". Electronic News. 31. Sage Publications: 26 – via Gale.
  17. ^ Ristelhueber, Robert; Ira Sager (1986-09-08). "4 PC suppliers add 286 units in battle with compatibles". Electronic News. 32. Sage Publications: 1 et seq. – via Gale.
  18. ^ a b Whitmore, Sam (1986-01-14). "IBM Debuts PC AT-Based Point-of-Sale Retail System". PC Week. 3 (2). Ziff-Davis: 8 – via Gale.
  19. ^ a b Whitmore, Sam (1986-04-01). "IBM meetings said to key on new products". PC Week. 3 (13). Ziff-Davis: 1 – via Gale.
  20. ^ a b c d "IBM Gives Up on the Personal XT/, AT/370". Computer Business Review. New Statesman Media Group. 1987-04-13. Archived from the original on 2023-09-20.
  21. ^ Sandker, C. (1985-10-08). "IBM Delivers New PC AT Featuring 30M-Byte Disk: Sets a Price Tag of $5,995". PC Week. 2 (40). Ziff-Davis: 1 – via Gale.
  22. ^ "Desktop Systems". PC Tech Journal. 4 (13). Ziff-Davis: 69. 1987 – via the Internet Archive.
  23. ^ a b Kennedy, Don (1986-04-02). "IBM has boosted the processor speed and storage capacity of its personal computer models". PR Newswire. p. NYPR14C – via Gale.
  24. ^ Scott M. Mueller (2011). Upgrading and Repairing PCs (20th ed.). Que Publishing. p. 882. ISBN 978-0-13-268218-3.
  25. ^ IBM's official 1986 response to "What percentage of the 20 MB drives in PC ATs have failed?" was "We consider that information to be confidential. However, based on the several customer surveys on the AT that we have conducted for IBM, an overwhelming percentage of AT owners tell us they're satisfied with the system." (questions on page 110, answers on page 111, PC Magazine, April 29, 1986). The article's opening sentence, which reads "If you own an IBM PC AT and your hard disk hasn't crashed yet, don't worry -- it probably will." Archived November 4, 2012, at the Wayback Machine was described as "a rarity in computer journalism" by the Chicago Sun-Times and the Sun-Times called it a "badly flawed 20-megabyte" disk drive.
  26. ^ "HP Computer Museum". hpmuseum dot net. BGImages Australia. Retrieved 2020-07-09.
  27. ^ Zenith Data Systems (August 1989). "Zenith Innovates Again". PC Magazine. p. 375 – via Google Books.
  28. ^ "Equity II+ Product Info" (PDF). Epson USA. Retrieved 2020-07-08.
  29. ^ "Equity III Product Info" (PDF). Epson USA. Retrieved 2020-07-08.
  30. ^ Sanger, David E. (1985-01-18). "Computer Giant Finds Problems in Success". The New York Times. Retrieved 2011-02-25.
  31. ^ Ahl, David H. (December 1984). "Top 12 computers of 1984". Creative Computing. Retrieved 2019-03-16.
  32. ^ Jeffery, Brian (1985-09-30). "IBM's high-end micros encroaching on mini territory". Computerworld. pp. SR/20–21. Retrieved 2015-01-02.
  33. ^ Potts, Mark (1987-04-03). "New IBM Computers Unveiled". The Washington Post. p. F1 – via ProQuest.
  34. ^ "IBM Adding PS/2 Computer Compatible with AT Models". Sun-Sentinel: 3D. 1988-09-14 – via ProQuest. {{cite journal}}: Unknown parameter |agency= ignored (help)
  35. ^ Miller, Michael W. (1988-09-14). "IBM Introduces Low-end PC Aimed at Single Users". The Wall Street Journal. Dow Jones & Company: 1 – via ProQuest.
  36. ^ Rosch, Winn L. (1989-01-17). "What's Old Is New". PC Magazine. 8 (1). Ziff-Davis: 259–272 – via Google Books.
  • IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
  • PC AT entry at