Editing IBM Personal Computer AT

{{Short description|IBM personal computer released in 1984}}
{{Use mdy dates|date=May 2024|cs1-dates=y}}
{{Infobox information appliance
| name = IBM PC/AT (System Unit 5170)
| title = 
| aka = IBM AT, PC/AT
| logo = IBM Personal Computer AT badge recreation.svg
| logo_upright = 0.5
| image = File:IBM PC AT.jpg
| caption = 
| developer = 
| manufacturer = IBM
| family = 
| type = [[Personal Computer]]
| generation = 
| releasedate = {{Start date and age|1984|08|14}}<ref>{{cite magazine |last1=Somerson |first1=Paul |title=AT the Party |magazine=PC Magazine |date=November 13, 1984 |page=123 |url=https://books.google.com/books?id=-Ukz6hjZEA4C&pg=PA117 |access-date=July 5, 2014|ref=PC Magazine, November 13, 1984, Page 123}}</ref>
| lifespan = 
| price = Approx. {{US$|6000|1983|round=-2}}
| discontinued = April 2, 1987<ref name="vintage">{{Cite web |url=http://www.vintage-computer.com/ibmpcat.shtml |title=IBM PC AT at Vintage Computer |access-date=October 20, 2010  |archive-date=August 22, 2020  |archive-url=https://web.archive.org/web/20200822055008/http://www.vintage-computer.com/ibmpcat.shtml |url-status=dead }}</ref>
| unitssold = >100,000
| unitsshipped = 
| media = 
| os = {{ubl|[[IBM PC DOS]] 3.0+|OS/2 1.x|[[Interactive Systems Corporation#PC.2FIX|PC/IX]] 1.1|IBM & SCO [[Xenix]]|[[Windows]] 1.0–3.1}}
| power = 120/240 VAC
| soc = 
| cpu = [[Intel 80286]] @ 6 and 8&nbsp;MHz
| memory = 256–512&nbsp;[[Kilobyte|KB]] onboard + 3.5&nbsp;[[Megabyte|MB]] with additional memory cards
| storage = 20&nbsp;MB hard drive<br/>1.2&nbsp;MB HD 5.25" (135&nbsp;mm) floppy drive
| memory card = 
| display = 
| graphics = [[Color Graphics Adapter|CGA]], [[Enhanced Graphics Adapter|EGA]]
| sound = 
| input = Parallel, serial
| controllers = 
| camera = 
| touchpad = 
| connectivity = 
| platform = 
| service = 
| dimensions = 
| weight = 
| topgame = 
| compatibility = 
| predecessor = [[IBM Personal Computer XT]]
| successor = [[IBM Personal System/2]]
| related = [[List of IBM Personal Computer models]]
}}

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 Personal Computer/XT|IBM PC/XT]] and its [[IBM Portable Personal Computer|IBM Portable PC]] variant. It was designed around the [[Intel 80286]] [[microprocessor]].

== Name ==
IBM did not specify an expanded form of ''AT'' on the machine, press releases, brochures or documentation, but some sources<ref name="sanger19841119" /> expand the term as ''Advanced Technology'', including at least one internal IBM document.<ref>{{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}}</ref>

==History==
[[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.<ref name="sanger19841119">{{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}}</ref>
[[File:IBM 5170 with Floor Standing Enclosure.jpg|thumb|IBM 5170 with Floor Standing Enclosure<ref>{{Cite web |title=IBM PC-AT - UvA Computer Museum catalogue |url=https://ub.fnwi.uva.nl/computermuseum/pcat.html#close |access-date=July 15, 2023 |website=[[University of Amsterdam]]}}</ref> and compatible non-IBM display]]

== Features ==
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 [[x87#80287|Intel 80287]], for faster execution of [[Floating-point arithmetic|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 [[Interrupt request (PC architecture)|IRQs]] and seven [[Direct memory access|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.<ref>{{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}}</ref><ref>{{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}}</ref> 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&nbsp;[[Megabyte|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.<ref name="Horan2013">{{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}}</ref><ref name="ShanleyAnderson1995">{{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}}</ref> 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&nbsp;Hz timer (on IRQ 8), a much finer resolution than the 18&nbsp;Hz timer on the PC.<ref name="Austerlitz2002">{{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}}</ref>

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 switch]]es used to set system settings on PCs. Most AT clones have the setup program in [[Read-only memory|ROM]] rather than on disk.

=== Storage ===
The standard floppy drive was upgraded to a 1.2&nbsp;MB {{frac|5|1|4}}&nbsp;inch [[floppy disk drive]] (15 [[Disk sector|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|Problems]] below). {{frac|3|1|2}}&nbsp;inch floppy drives became available in later ATs.{{fact|date=January 2023}}

A 20&nbsp;MB [[hard disk drive]] was included as standard. Early drives were manufactured by [[Computer Memories Inc.|Computer Memories]] and were found to be very unreliable.<ref name="dickinson19850625">{{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}}</ref>

=== Peripherals ===
The AT included the [[AT keyboard]], initially a new 84-key layout (the 84th key being [[System request|SysRq]]). The numerical keypad was now clearly separated from the main key group, and indicator [[light-emitting diode|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 code]]s. 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 [[computer lock|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 [[Color Graphics Adapter|CGA]], [[IBM Monochrome Display Adapter|MDA]], [[Enhanced Graphics Adapter|EGA]], or [[Professional Graphics Controller|PGA]] video cards.

The [[8250 UART]] from the PC was upgraded to the [[Universal asynchronous receiver/transmitter#UART models|16450]], but since both chips had single-byte buffers, high-speed serial communication was problematic as with the XT.<ref name="NickallsRamasubramanian1995">{{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]}}</ref>

=== Models ===
{{Main|List of IBM Personal Computer models}}
{| class="wikitable sortable sticky-header sort-under" 
|- 
|+ IBM Personal Computer models
! Type
! IBM {{abbr|P/N|part number}}
! Date announced
! Date withdrawn
! Bus
! {{abbr|No.|Number}} of<br />slots
! {{abbr|No.|Number}} of<br />bays
! Processor          
! Clock<br />speed<br />{{small|(MHz)}}
! Stock onboard RAM<br />{{small|(KB)}}
! Maximum onboard RAM<br />{{small|(KB)}}
! {{abbr|FDD|Floppy disk drive}}
! {{abbr|HDD|Hard disk drive}}
! Notes
! Ref(s).
|-
| AT
| 5170-068
| {{date table sorting|1984|08|14|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 256&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| none
|  
| <ref name=ibmref40>{{cite book | date=April 6, 1987 | url=https://bitsavers.org/pdf/ibm/pc/PS2_and_IBM_PC_Product_Reference_Version_4.0_Apr87.pdf#page=244 | title=IBM Personal System/2 and IBM Personal Computer Product Reference | edition=4.0 | publisher=International Business Machines Corporation | via=[[Bitsavers]]}}</ref>{{rp|49}}<ref name=pcweekatannounced>{{cite journal | last=Whitmore | first=Sam | date=August 14, 1984 | url=https://link.gale.com/apps/doc/A573108/GPS?sid=wikipedia | title=IBM's New Generation: The PC AT: AT Hikes CPU Speed Sixfold, Memory Fivefold | journal=PC Week | publisher=Ziff-Davis | volume=1 | issue=32 | page=1 | via=Gale | quote=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.}}</ref><ref name=atdiscontinued>{{cite journal | last=Abruzzese | first=George | date=June 29, 1987 | url=https://books.google.com/books?id=xjAEAAAAMBAJ&pg=PA1 | title=IBM Plans to Terminate Remainder of PC Line | journal=InfoWorld | publisher=IDG Publications | volume=9 | issue=26 | pages=1, 69 | via=Google Books}}</ref>
|-
| AT
| 5170-099
| {{date table sorting|1984|08|14|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 20&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=pcweekatannounced /><ref name=atdiscontinued />
|-
| AT
| 5170-239
| {{date table sorting|1985|10|01|format=my}}
| {{date table sorting|1986|09|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=at30mb>{{cite journal | last=Kuzbyt | first=Bob | date=October 7, 1985 | url=https://link.gale.com/apps/doc/A3972202/GPS?sid=wikipedia | title=IBM introduces 30-MB 5.25-inch Winchester disk drive for AT | journal=Electronic News | publisher= Sage Publications | volume=31 | page=26 | via=Gale}}</ref><ref name=Ristelhueber>{{cite journal | last=Ristelhueber | first=Robert | author2=Ira Sager | date=September 8, 1986 | url=https://link.gale.com/apps/doc/A4398861/GPS?sid=wikipedia | title=4 PC suppliers add 286 units in battle with compatibles | journal=Electronic News | publisher=Sage Publications | volume=32 | pages=1 ''et seq'' | via=Gale}}</ref>
|-
| AT
| 5170-839
| {{date table sorting|1986|01|08|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| two&nbsp;30&nbsp;MB
| Includes controllers for the [[IBM 4680|IBM 4680 Store System]]
| <ref name=ibmref40 />{{rp|236}}<ref name=at4680>{{cite journal | last=Whitmore | first=Sam | date=January 14, 1986 | url=https://link.gale.com/apps/doc/A656864/GPS?sid=wikipedia | title=IBM Debuts PC AT-Based Point-of-Sale Retail System | journal=PC Week | publisher=Ziff-Davis | volume=3 | issue=2 | page=8 | via=Gale}}</ref><ref name=atdiscontinued />
|-
| AT
| 5170-899
| {{date table sorting|1986|01|08|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| two&nbsp;20&nbsp;MB
| Includes controllers for the [[IBM 4680|IBM 4680 Store System]]
| <ref name=ibmref40 />{{rp|236}}<ref name=at4680 /><ref name=atdiscontinued />
|-
| AT
| 5170-319
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=at8mhz>{{cite journal | last=Whitmore | first=Sam | date=April 1, 1986 | url=https://link.gale.com/apps/doc/A666974/GPS?sid=wikipedia | title=IBM meetings said to key on new products | journal=PC Week | publisher=Ziff-Davis | volume=3 | issue=13 | page=1 | via=Gale}}</ref><ref name=atdiscontinued />
|-
| AT
| 5170-339
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|07|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| Shipped with Enhanced Keyboard
| <ref name=ibmref40 />{{rp|236}}<ref name=at8mhz /><ref name=atdiscontinued />
|-
| AT
| 5171-168
| {{date table sorting|1986|10|21|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| none
| Built to [[Tempest (codename)|TEMPEST]] specifications
| <ref name=ibmref40 />{{rp|64}}<ref name=atdiscontinued />
|-
| AT
| 5171-339
| {{date table sorting|1986|10|21|format=my}}
| {{date table sorting|1987|07|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| Built to [[Tempest (codename)|TEMPEST]] specifications
| <ref name=ibmref40 />{{rp|64}}<ref name=atdiscontinued />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-599
| {{date table sorting|1984|10|25|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 20&nbsp;MB
| 
| <ref name=ibmref40 />{{rp|202}}<ref name=givesup>{{cite journal | date=April 13, 1987 | url=https://techmonitor.ai/technology/ibm_gives_up_on_the_personal_xt_at370 | title=IBM Gives Up on the Personal XT/, AT/370 | journal=Computer Business Review | publisher=New Statesman Media Group | archiveurl=https://archive.today/20230920021426/https://techmonitor.ai/technology/ibm_gives_up_on_the_personal_xt_at370 | archivedate=September 20, 2023}}</ref>
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-739
| {{date table sorting|1985|10|01|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|202}}<ref name=sandker>{{cite journal | last=Sandker | first=C. | date=October 8, 1985 | url=https://link.gale.com/apps/doc/A649986/GPS?sid=wikipedia | title=IBM Delivers New PC AT Featuring 30M-Byte Disk: Sets a Price Tag of $5,995 | journal=PC Week | publisher=Ziff-Davis | volume=2 | issue=40 | page=1 | via=Gale}}</ref><ref name=pctjv4n13>{{cite journal | date=1987 | url=https://archive.org/details/PC_Tech_Journal_vol04_n13/page/69/ | title=Desktop Systems | journal=PC Tech Journal | publisher=Ziff-Davis | volume=4 | issue=13 | page=69 | via=the Internet Archive}}</ref><ref name=givesup />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-919
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| 1&nbsp;MB total RAM (peripheral)
| <ref name=ibmref40 />{{rp|57}}<ref name=prnewswireat370>{{cite web | last=Kennedy | first=Don | date=April 2, 1986 | url=https://link.gale.com/apps/doc/A4196480/GPS?sid=wikipedia | title=IBM has boosted the processor speed and storage capacity of its personal computer models | publisher=PR Newswire | page=NYPR14C | via=Gale}}</ref><ref name=givesup />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-939
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| 1&nbsp;MB total RAM (peripheral); shipped with Enhanced Keyboard
| <ref name=ibmref40 />{{rp|57}}<ref name=prnewswireat370/><ref name=givesup />
|}

=== 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&nbsp;V line and a minimum of 2.5 amperes on its +12&nbsp;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&nbsp;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&nbsp;V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (dissipating 28.8 watts), getting fairly hot.<ref name="Mueller2011">{{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}}</ref>

== Problems ==
In addition to the unreliable hard disk drive,<ref>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.</ref> 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&nbsp;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&nbsp;KB disk drive faceplate. If the user accidentally used a high-density diskette in the 360&nbsp;KB drive, it would sometimes work, for a while, but the high-[[coercivity]] oxide would take a very weak magnetization from the 360&nbsp;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&nbsp;KB drive, so they were incapable of fully erasing and overwriting tracks written by a 360&nbsp;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.

== Clones ==
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&nbsp;MB {{frac|5|1|4}} inch floppy disk drives and other defining technical features of the IBM PC AT.

Large companies that had been late to release PC compatibles rushed out AT compatibles, with an estimated 100,000 sold by the end of 1985.<ref name="bright19860623">{{Cite magazine |last=Bright |first=David |date=1986-06-23 |title=AT compatibles: Systems vendors push to give single-source option |url=https://books.google.com/books?id=mop2UUCt4kIC&pg=PA1#v=onepage&q&f=true |access-date=2025-05-17 |magazine=Computerworld |pages=1, 14-15 |volume=XX |issue=25}}</ref> In the United States, popular brands of AT clones included the [[Tandy 3000]], [[Compaq Deskpro]] 286, [[Hewlett-Packard|HP]] Vectra,<ref>{{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}}</ref> [[Zenith Data Systems#Computers|Zenith]] Z-286,<ref>{{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}}</ref> [[Epson Equity]] Models II+<ref>{{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}}</ref> and III,<ref>{{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}}</ref> and [[Commodore PC compatible systems#Models|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 ATA|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 ([[Parallel ATA#IDE and ATA-1|Integrated Drive Electronics]]).

==Reception==
By January 1985, AT sales were so strong that IBM and its suppliers could not keep up with demand.<ref name="sanger19850118">{{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}}</ref> An estimated 350,000 were sold by the end of 1985.{{r|bright19860623}}

''[[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".<ref name="lockwood198412">{{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}}</ref> 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".<ref name="ahl198412">{{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}}</ref> 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.<ref name="jeffery19850930">{{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}}</ref>

==Successor==
{{main|IBM PS/2 Model 30 286}}
On April 2, 1987, IBM announced the [[IBM Personal System/2|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.<ref name=Potts1987>{{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}}</ref> 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.<ref name=atdiscontinued/> While the PC/XT received a directly compatible replacement in the form of the [[IBM PS/2 Model 30|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 [[IBM PS/2 Model 30 286|PS/2 Model 30 286]], which featured an Intel 80286 processor and 16-bit ISA expansion slots,<ref name=Reuters1988>{{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}}</ref><ref name=Miller1988>{{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}}</ref> serving as the direct replacement for the AT for customers who wanted to buy a true IBM system.<ref name=Rosch1989>{{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}}</ref>

==Timeline==
{{Timeline of the IBM Personal Computer}}

== See also ==
* [[AT (form factor)]]
* [[Industry Standard Architecture]]
* [[IBM PS/2]]

== References ==
{{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}}

==External links==
* [https://books.google.com/books?id=-Ukz6hjZEA4C&pg=PA117 Cover story: "IBM brings out the big guns"], ''PC Mag'' November 13, 1984, pp.&nbsp;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}}

[[Category:X86 IBM personal computers|Personal Computer AT]]
[[Category:16-bit computers]]
[[Category:Computer-related introductions in 1984]]