IBM Personal Computer AT

IBM did not specify an expanded form of AT on the machine, press releases, brochures or documentation, but some sources expand the term as Advanced Technology, including at least one internal IBM document.{{Cite book|url=http://www.bitsavers.org/pdf/ibm/pc/IBM_Personal_System_2_and_IBM_Personal_Computer_Publication_Reference_Jan89.pdf|title=IBM Personal System 2 and IBM Personal Computer Publication Reference Jan89|year=1989|pages=66}}

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.{{Cite news |url=https://www.nytimes.com/1984/11/19/business/ibm-entry-unchallenged-at-show.html |title=I.B.M. Entry Unchallenged at Show |last=Sanger |first=David E. |date=November 19, 1984 |work=The New York Times |access-date=July 3, 2017 |language=en-US |issn=0362-4331}}

File:IBM 5170 with Floor Standing Enclosure.jpg}} 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 introduced 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.{{cite magazine |last=Derfler |first=Frank J. Jr. |url=https://books.google.com/books?id=-Xr7Ic-ivyMC&pg=PT53|title=Tutor |magazine = PC Magazine |date=December 26, 1989|publisher=Ziff Davis |page=53 |issn=0888-8507}}{{cite book|first=N. |last=Mathivanan |title=PC-Based Instrumentation: Concepts and Practice |url=https://books.google.com/books?id=OB65rMNQDo8C&pg=PA227 |year=2007 |publisher=PHI Learning |isbn=978-81-203-3076-4 |pages=227–229}} 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.{{cite book|author=Brendan Horan|url=https://books.google.com/books?id=PSonZP4Nj5sC&pg=PA146|title=Practical Raspberry Pi|date=March 26, 2013|publisher=Apress|isbn=978-1-4302-4972-6|page=146}}{{cite book|author-last1=Shanley|author-first1=Tom|url=https://archive.org/details/ISA_System_Architecture|title=ISA System Architecture|author-last2=Anderson|author-first2=Don|date=1995|publisher=Mindshare, Inc. / Addison-Wesley Publishing Company|isbn=0-201-40996-8|editor-last=Swindle|editor-first=John|edition=3|pages=[https://archive.org/details/ISA_System_Architecture/page/n451 441]–444}} 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.{{cite book|author=Howard Austerlitz|url=https://books.google.com/books?id=gK4DxMaqmYYC&pg=PA90|title=Data Acquisition Techniques Using PCs|publisher=Academic Press|year=2002|isbn=978-0-08-053025-3|pages=90–91}}

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 {{frac|5|1|4}} 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). {{frac|3|1|2}} inch floppy drives became available in later ATs.{{fact|date=January 2023}}

A 20 MB hard disk drive was included as standard. Early drives were manufactured by Computer Memories and were found to be very unreliable.{{cite news|author=Dickinson, John|date=June 25, 1985|title=The AT's Slipped Disk|pages=55|work=PC Magazine|url=https://books.google.com/books?id=uqInkwnBdacC&pg=PA55|access-date=October 28, 2013}}

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.{{cite book|first1=Richard W. D. |last1=Nickalls |first2=R. |last2=Ramasubramanian |title=Interfacing the IBM-PC to Medical Equipment: The Art of Serial Communication |url=https://archive.org/details/interfacingibmpc0000nick |url-access=registration |year=1995 |publisher=Cambridge University Press |isbn=978-0-521-46280-8|page=[https://archive.org/details/interfacingibmpc0000nick/page/25 25]}}

{{Main|List of IBM Personal Computer models}}

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.{{cite book|author=Scott M. Mueller|title=Upgrading and Repairing PCs|year=2011|publisher=Que Publishing|isbn=978-0-13-268218-3|page=882|edition=20th}}

In addition to the unreliable hard disk drive,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." {{Cite web |url=http://www.highbeam.com/doc/1P2-3760999.html |title=Archived copy |access-date=2010-06-30 |archive-date=2012-11-04 |archive-url=https://web.archive.org/web/20121104075857/http://www.highbeam.com/doc/1P2-3760999.html |url-status=dead }} was described as "a rarity in computer journalism" by the Chicago Sun-Times http://www.encyclopedia.com/doc/1P2-3760999.html and the Sun-Times called it a "badly flawed 20-megabyte" 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 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|date=February 2021}} 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 {{frac|5|1|4}} 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,{{cite web |title=HP Computer Museum |url=http://hpmuseum.net/display_item.php?hw=125 |website=hpmuseum dot net |publisher=BGImages Australia |access-date=July 9, 2020}} Zenith Z-286,{{cite magazine |title=Zenith Innovates Again |magazine=PC Magazine |date=August 1989 |page=375 |url=https://books.google.com/books?id=fSMW_RgWwIgC&q=zenith+z-286&pg=PT362 |via=Google Books |author=Zenith Data Systems}} Epson Equity Models II+{{cite web |title=Equity II+ Product Info |url=https://files.support.epson.com/pdf/e2p___/e2p___pg.pdf |website=files.support.epson.com |publisher=Epson USA |access-date=July 8, 2020}} and III,{{cite web |title=Equity III Product Info |url=http://files.support.epson.com/pdf/e3p___/e3p___pg.pdf|website=files.support.epson.com |publisher=Epson USA |access-date=July 8, 2020}} 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 for the AT, originally a Seagate ST506 compatible interface on IBM's disk controller card, was updated and standardized as ATA ("AT Attachment") by Western Digital and Compaq in 1986, and later renamed PATA (Parallel AT Attachment). The ATA interface was also known as IDE, because the drive controller, instead of being on the interface card, was integrated into the drive (Integrated Drive Electronics).

By January 1985, AT sales were so strong that IBM and its suppliers could not keep up with demand.{{cite news | url=http://www.nytimes.com/1985/01/18/business/computer-giant-finds-problems-in-success.html?pagewanted=all | title=Computer Giant Finds Problems in Success | access-date=February 25, 2011 | author=Sanger, David E. | date=January 18, 1985 | work=The New York Times}}

Creative Computing in December 1984 said "the folks at IBM have done it again". It approved of the "refreshingly standard" keyboard layout, but said that "for most individuals, AT power represents overkill ... a state of the art machine at a top of the line price".{{Cite magazine |last=Lockwood |first=Russ |date=December 1984 |title=IBM PC AT; state of the art technology enhances the PC. |url=https://www.atarimagazines.com/creative/v10n12/32_IBM_PC_AT_state_of_the_a.php |access-date=2025-01-31 |magazine=Creative Computing |page=32}} That month the magazine's David H. Ahl 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".{{Cite magazine |last=Ahl |first=David H. |author-link=David H. Ahl |date=December 1984 |title=Top 12 computers of 1984 |url=https://www.atarimagazines.com/creative/v10n12/10_Top_12_computers_of_1984.php |magazine=Creative Computing |access-date=March 16, 2019}} 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.{{cite news | url=https://books.google.com/books?id=MmR2XHzI9yoC&pg=RA1-PA46 | title=IBM's high-end micros encroaching on mini territory | work=Computerworld | date=September 30, 1985 | access-date=January 2, 2015 | author=Jeffery, Brian | pages=SR/20–21}}

{{main|IBM PS/2 Model 30 286}}

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.{{cite news | last=Potts | first=Mark | date=April 3, 1987 | url=https://www.proquest.com/docview/306890384/ | title=New IBM Computers Unveiled | newspaper=The Washington Post | page=F1 | via=ProQuest}} 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. 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,{{cite news | agency=Reuters | date=September 14, 1988 | url=https://www.proquest.com/docview/389348570/ | title=IBM Adding PS/2 Computer Compatible with AT Models | work=Sun-Sentinel | page=3D | via=ProQuest}}{{cite journal | last=Miller | first=Michael W. | date=September 14, 1988 | url=https://www.proquest.com/docview/398059991/ | title=IBM Introduces Low-end PC Aimed at Single Users | journal=The Wall Street Journal | publisher=Dow Jones & Company | page=1 | via=ProQuest}} serving as the direct replacement for the AT for customers who wanted to buy a true IBM system.{{cite journal | last=Rosch | first=Winn L. | date=January 17, 1989 | url=https://books.google.com/books?id=C6VFJIbxX7MC&pg=PA267 | title=What's Old Is New | journal=PC Magazine | publisher=Ziff-Davis | volume=8 | issue=1 | pages=259–272 | via=Google Books}}

{{Timeline of the IBM Personal Computer}}

• AT (form factor)
• Industry Standard Architecture
• IBM PS/2

{{Reflist}}

;Notes

{{Refbegin}}

• IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
• [https://web.archive.org/web/20130316131729/http://www.old-computers.com/museum/computer.asp?st=1&c=185 PC AT entry at old-computers.com]

{{Refend}}

• [https://books.google.com/books?id=-Ukz6hjZEA4C&pg=PA117 Cover story: "IBM brings out the big guns"], PC Mag November 13, 1984, pp. 117–133
• [https://web.archive.org/web/20160305180256/http://vintage-computer.com/vcforum/showthread.php?26480-ibm-pc-at-(5170) Wiki entry for PC AT at the Vintage Computer Forums]
• [http://historycorner.de/CoCo3/IBM/ibm5170AT.html Historycorner.de – The IBM PC AT (IBM 5170)] {{In lang|de}}
• [http://www.minuszerodegrees.net IBM 5170 information at www.minuszerodegrees.net]

{{IBM personal computers}}

{{Authority control}}

Personal Computer AT

Category:16-bit computers

Category:Computer-related introductions in 1984