Hubbry Logo
IBM Personal Computer ATIBM Personal Computer ATMain
Open search
IBM Personal Computer AT
Community hub
IBM Personal Computer AT
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
IBM Personal Computer AT
IBM Personal Computer AT
IBM Personal Computer AT
View on Wikipedia
from Wikipedia

IBM PC AT (System Unit 5170)
Also known asIBM AT, PC/AT
ManufacturerIBM
TypePersonal Computer
Release dateAugust 14, 1984; 41 years ago (1984-08-14)[1]
Lifespan1984–1987
Introductory priceApprox. US$6,000 (equivalent to $18,900 in 2024)
DiscontinuedApril 2, 1987[2]
Units sold>100,000
Operating system
CPUIntel 80286 @ 6 and 8 MHz
Memory256–512 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 Personal System/2
RelatedList 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 PC XT and its IBM Portable PC variant. It was designed around the Intel 80286 microprocessor.

Name

[edit]

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]

History

[edit]

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

Features

[edit]

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.[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.

Storage

[edit]

The standard floppy drive was upgraded to a 1.2 MB 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). 3+1⁄2 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]

Peripherals

[edit]

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]

Models

[edit]
Main article: List of IBM Personal Computer models
IBM Personal Computer AT models
Type
IBM P/N
 Date announced Date withdrawn Bus
No. of slots
No. of bays
 Processor
Clock speed
(MHz)
Stock onboard RAM
(KB)
Maximum onboard RAM
(KB)
 FDD HDD Notes Ref(s).
AT
5170-068
 August 1984 June 1987 ISA, 16-bit 8 3 Intel 80286 6 256 KB 512 KB 1.2 MB none  
AT
5170-099
 August 1984 June 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 20 MB  
AT
5170-239
 October 1985 September 1986 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 30 MB  
AT
5170-839
 January 1986 June 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB two 30 MB Includes controllers for the IBM 4680 Store System
AT
5170-899
 January 1986 June 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB two 20 MB Includes controllers for the IBM 4680 Store System
AT
5170-319
 April 1986 June 1987 ISA, 16-bit 8 3 Intel 80286 8 512 KB 512 KB 1.2 MB 30 MB  
AT
5170-339
 April 1986 July 1987 ISA, 16-bit 8 3 Intel 80286 8 512 KB 512 KB 1.2 MB 30 MB Shipped with Enhanced Keyboard
AT
5171-168
 October 1986 June 1987 ISA, 16-bit 8 3 Intel 80286 8 512 KB 512 KB 1.2 MB none Built to TEMPEST specifications
AT
5171-339
 October 1986 July 1987 ISA, 16-bit 8 3 Intel 80286 8 512 KB 512 KB 1.2 MB 30 MB Built to TEMPEST specifications
5170-599
 October 1984 April 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 20 MB
5170-739
 October 1985 April 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 30 MB  
5170-919
 April 1986 April 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 30 MB 1 MB total RAM (peripheral)
5170-939
 April 1986 April 1987 ISA, 16-bit 8 3 Intel 80286 6 512 KB 512 KB 1.2 MB 30 MB 1 MB total RAM (peripheral); shipped with Enhanced Keyboard

Power supply

[edit]

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]

Problems

[edit]

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.

Clones

[edit]

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+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.[26] In the United States, popular brands of AT clones included the Tandy 3000, Compaq Deskpro 286, HP Vectra,[27] Zenith Z-286,[28] Epson Equity Models II+[29] and III,[30] 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).

Reception

[edit]

By January 1985, AT sales were so strong that IBM and its suppliers could not keep up with demand.[31] An estimated 350,000 were sold by the end of 1985.[26]

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".[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".[33] 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.[34]

Successor

[edit]
Main article: 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.[35] 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,[36][37] serving as the direct replacement for the AT for customers who wanted to buy a true IBM system.[38]

Timeline

[edit]
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 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

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

IBM Personal Computer AT

View on Grokipedia
from Grokipedia
The IBM Personal Computer AT (model 5170) was a 16-bit personal computer developed and released by IBM in 1984 as the successor to the IBM PC XT, featuring the Intel 80286 microprocessor operating at 6 MHz, 256 KB or 512 KB of base RAM expandable to 3 MB or more via add-on cards, a standard 1.2 MB high-density 5.25-inch floppy disk drive, and an optional 20 MB fixed hard disk drive, all housed in a desktop chassis with eight expansion slots supporting the new 16-bit ISA bus architecture. Designed primarily for professional, business, and multitasking applications, it provided approximately twice the processing speed of the 8-bit IBM PC XT while maintaining full backward compatibility with existing IBM PC software, peripherals, and the MS-DOS operating system (version 3.0 and later). Announced on August 14, 1984, with first shipments in September, the initial models included the base Model 68 (256 KB RAM, no hard drive, priced at $3,995) and the upgraded Model 99 (512 KB RAM, 20 MB hard drive, priced at $5,795), both utilizing a Type 1 motherboard and supporting an optional Intel 80287 math coprocessor for enhanced numeric processing. Key innovations included a battery-backed for date and time management, an enhanced 84-key keyboard with separate cursor and function keys, a keylock for system security, and integrated diagnostics via beep codes and error reporting, all integrated on the system board to simplify for programmers through the . The system's power supply delivered 192 watts, supporting up to 450 VA input, and it featured seven DMA channels (four 8-bit, three 16-bit) and 16 interrupt levels via dual 8259A controllers for efficient I/O handling of devices like serial/parallel ports, fixed disks, and diskettes. Subsequent updates expanded the lineup: in October 1985, Model 239 introduced a Type 2 motherboard with a 30 MB hard drive option; and in April 1986, Models 319 and 339 upgraded to an 8 MHz clock speed, 512 KB base RAM, 30 MB hard drive, and—for the 339—an enhanced 101-key keyboard divided into typing, numeric, cursor, and function sections with status indicators. Memory addressing supported 1 MB in real mode and up to 16 MB physical (1 GB virtual per task) in protected mode, with 640 KB conventional RAM and extended memory beyond 1 MB, enabling advanced features like multitasking and memory protection not feasible on prior 8-bit systems. Storage interfaces allowed concurrent operations on up to two fixed drives (up to 1,024 cylinders, 16 heads) and two diskette drives, with error correction via ECC for reliability in professional environments. The PC AT's , including its 16-bit bus and standardized expansion capabilities, established the "AT" form factor and compatibility standard that influenced personal computing design for years, supporting networking, high-level languages like BASIC and , and adapters while operating in environments from 10–43°C with low noise output of 42 dB. Production continued until April 1987, with approximately 900,000 units sold, paving the way for the line while remaining a benchmark for expandable, high-performance desktop systems.

Development and Release

Naming and Origins

The designation "AT" for the IBM Personal Computer AT is widely understood to stand for "Advanced Technology," reflecting its advancements over prior models, though IBM did not formally expand the acronym in public documentation. This naming emphasized the system's incorporation of cutting-edge features, positioning it as a significant in IBM's personal computing lineup. Development of the PC AT originated in 1982–1983 within IBM's Boca Raton facility, initially as a led by Don Estridge, building on the commercial success of the IBM PC (1981) and its XT extension (1983). The initiative responded to the rapid growth in software demands, exemplified by applications like , which strained the capabilities of earlier 8088-based systems and necessitated higher performance for business users. Key efforts focused on integrating Intel's 80286 processor to enable expanded memory addressing and faster processing, marking a shift toward 16-bit architecture. The project fell under IBM's Entry Systems Division (ESD), formalized on August 1, 1983, which absorbed the PC development team and scaled operations amid increasing competition from clones. At launch in September 1984, IBM priced the base model between US$4,000 and $6,000, equivalent to approximately $12,000–$18,000 in dollars, targeting and enterprise markets while balancing affordability with premium positioning.

Announcement and Market Entry

IBM announced the Personal Computer AT on August 14, 1984, via official Announcement Letter 184-097, marking its entry into the high-performance personal computing market. The system, referred to as "Advanced Technology," was unveiled as a significant upgrade for professional and corporate environments, with first customer shipments scheduled for September 14, 1984. emphasized its enhanced processing power—more than twice that of the PC XT—through the microprocessor, which supported multitasking for improved productivity in office applications. At launch, the PC AT was offered in two primary configurations to suit varying needs: Model 68 with 256 KB base RAM and a 1.2 MB 5.25-inch floppy drive, expandable to 512 KB, and Model 99 with 512 KB RAM, the floppy drive, and an optional 20 MB for faster data access. Systems were bundled with PC DOS 3.00, along with the "Exploring the IBM Personal Computer AT" software to aid users in leveraging its capabilities. This setup targeted corporate users seeking reliability and expandability, including eight ISA expansion slots for peripherals and an integrated battery-backed clock/calendar. The market entry strategy focused on positioning the PC AT as the premier choice for business multitasking and data-intensive tasks, differentiating it from consumer-oriented systems like the PCjr.

Technical Specifications

Processor and Memory

The IBM Personal Computer AT utilized the as its , a 16-bit capable of operating in both and . In , it maintained compatibility with the 8086/8088 , supporting up to 1 MB of contiguous addressing through 20-bit addresses organized in 64 KB segments. introduced advanced features such as with four privilege levels, addressing up to 1 GB, and physical addressing limited to 16 MB via 24-bit addresses, enabling more robust multitasking and larger application support. The processor interfaced with a 16-bit data bus, facilitating faster and I/O operations compared to the 8-bit bus of prior models. The base clock speed of the 80286 was 6 MHz, corresponding to a 167 ns clock cycle, with bus cycles timed at 500 ns for 16-bit accesses. In 1986, IBM offered an optional upgrade to an 8 MHz 80286 variant, which increased processing throughput while maintaining compatibility with the system's architecture. This configuration delivered approximately 1.2 MIPS of performance at 6 MHz, roughly doubling the computational capability of the IBM PC XT's 8088 processor, which operated at around 0.6 MIPS under similar workloads. The enhanced efficiency stemmed from the 80286's improved instruction execution, including pipelined operations and reduced wait states for internal processing. Memory in the AT consisted of 256 KB or 512 KB of standard onboard dynamic RAM (DRAM), utilizing parity checking for single-bit error detection—employing an additional parity bit per byte (or two for 16-bit words)—to enhance reliability in business environments. This base memory formed the lower 640 KB conventional address space, with expansion possible up to 16 MB total through add-on cards in the system's slots, where extended memory began at 1 MB. Access times were optimized at one wait state for a three-clock-cycle read or write, and memory size was dynamically reported via BIOS interrupt 12h in 1 KB increments. Parity errors triggered interrupt 2, halting operations to prevent data corruption. A key innovation was the inclusion of a battery-backed (RTC) implemented with the MC146818 chip, marking the first such feature in an PC model and replacing manual date/time setup from boot. This provided timekeeping at whole-second resolution using a 32.768 kHz , along with alarm functions, periodic interrupts at rates up to 1.024 kHz, and 64 bytes of RAM for non-volatile storage of configuration , such as size, disk types, and flags. The RAM was accessed via I/O ports 70h (address) and 71h (), with a for integrity verification; low battery or errors prompted diagnostic messages like "Time & Date Not Set." The RTC operated independently of the main clock, using interrupt vector 8 (IRQ8) for updates, ensuring persistent setup across power cycles.

Expansion Bus and Peripherals

The IBM Personal Computer AT introduced the (ISA) bus, a 16-bit expansion bus designed to match the capabilities of the processor and enable greater system expandability compared to the 8-bit bus in prior models. This architecture featured 98-pin edge connectors on the motherboard's expansion slots, which integrated the original 62-pin 8-bit design with an additional 36 pins to support 16-bit data transfers, allowing for faster I/O operations and compatibility with a broader range of devices. The ISA bus operated at the system's clock speed, typically 6 MHz in early AT models and 8 MHz in later configurations, providing a theoretical maximum throughput of around 8 MB/s for 16-bit operations, though real-world performance was often lower due to protocol overhead. It supported 24-bit addressing, enabling access to up to 16 MB of total memory space, including the conventional 640 KB base memory limit for DOS applications and beyond that threshold for protected-mode operations. To maintain , the bus incorporated signals like IOCHRDY (I/O Channel Ready), which allowed 8-bit peripherals to insert wait states during 16-bit cycles, preventing timing mismatches and ensuring reliable operation with XT-era cards in the extended slots. Peripheral support was enhanced through the bus's expanded signaling, accommodating advanced graphics adapters such as the Monochrome Display Adapter (MDA) and for high-resolution monochrome text, alongside the for basic color graphics, and the new , which utilized the 16-bit pathway for up to 640x350 resolution with 16 simultaneous colors from a palette of 64. The motherboard included two serial ports () and one parallel port () directly connected to the bus, with expansion cards providing additional ports or specialized interfaces; the serial port also served as the primary connection for early devices, foreshadowing dedicated mouse ports in subsequent IBM systems. Power for these peripherals was distributed via dedicated pins on the 98-pin connectors, supplying +5 V (multiple pins for high current), +12 V, -5 V, and -12 V rails—offering more robust delivery than the XT's 62-pin slots, which had fewer power pins and lower capacity. Despite its innovations, the ISA bus exhibited limitations inherent to its design, including a maximum effective speed of 8 MHz in standard AT implementations, which introduced bottlenecks when paired with faster processors and required additional wait states for 8-bit devices that could halve transfer rates during mixed operations. These constraints highlighted the need for future bus evolutions, but the ISA remained a foundational standard for PC expandability into the .

Storage and Input/Output

The IBM Personal Computer AT incorporated advanced storage options designed to enhance data handling capabilities over its predecessor, the XT model. Standard configurations included a 1.2 MB high-density 5.25-inch floppy disk drive, capable of supporting up to 1.2 MB of formatted capacity using high-density media at 96 tracks per inch (TPI). This drive was incompatible with the XT's 360 KB double-density disks for high-capacity operations without adapters or media changes, as the high-density mechanism required specific signaling and diskette types to prevent data errors or drive damage. An optional hard disk drive provided 20 MB of storage using run-length limited (RLL) encoding on the ST-506 interface, allowing for denser data packing with 26 sectors per track compared to the modified frequency modulation (MFM) standard's 17 sectors, thereby increasing capacity while maintaining compatibility with existing controllers. The hard disk controller, implemented via the integrated Fixed Disk and Diskette Adapter, supported up to two ST-506/412 drives and a total storage capacity of 40 MB, facilitating reliable fixed-disk operations through 16-bit data transfers aligned with the AT's bus architecture. Input/output interfaces on the AT emphasized expandability and integration for peripheral connectivity. The system featured an integrated keyboard controller using the 8042 microcontroller, which managed bidirectional with the 84-key keyboard via a 5-pin and handled scan code processing through interrupt 9. Standard I/O provisions included two RS-232C for asynchronous communication (base addresses at hex 3F8-3FF and 2F8-2FF) and one for printer attachments (base address at hex 378-37F), enabling connections to modems, terminals, and printers without additional expansion cards in base models. A built-in speaker, driven by the channel 2 output at hex 61, provided basic for system events and diagnostics via simple tone generation, though no dedicated was included, limiting capabilities to beep-based alerts. The AT's power supply unit was a 192 W switched-mode design with fan cooling to mitigate the overheating problems observed in the original PC's linear supply, ensuring stable operation under load from the 80286 processor and peripherals. It delivered +5 V at 28 A for logic circuits, +12 V at 7 A for motors and drives, -5 V at 0.3 A for analog support, and -12 V at 0.8 A for serial port needs, with a "power good" signal to sequence startup and protect against voltage fluctuations. However, the supply required a minimum load (e.g., 7 A on +5 V and 2.5 A on +12 V) for regulation, which could lead to instability in lightly configured systems without additional resistors or drives, contributing to long-term reliability concerns in aging units.

Models and Configurations

Base Models

The IBM Personal Computer AT, designated model 5170, was announced on August 14, 1984, with first shipments in September. It utilized an microprocessor operating at 6 MHz. Base configurations were offered as Model 68 (256 KB RAM, no hard drive, priced at $3,995) or Model 99 (512 KB RAM, 20 MB , priced at $5,795), both using a Type 1 and including a 1.2 MB 5.25-inch high-capacity floppy drive alongside PC DOS 3.0 as the operating system. The design emphasized expandability through eight 16-bit ISA slots, supporting peripherals like serial/parallel adapters for connectivity. Graphics adapters such as MDA or CGA were optional via expansion cards. Later base variants included 8 MHz processor options before the model's phase-out. In October 1985, Model 239 was introduced with a Type 2 and a 30 MB hard drive option. Production of the model 5170 ran from 1984 until its discontinuation on April 2, 1987, with over 100,000 units manufactured. The system featured a full-size desktop form factor measuring 21.3 x 17.3 x 6.4 inches (width x depth x height) and a characteristic metal casing weighing approximately 37–43 pounds depending on drive configuration.

Optional Upgrades and Variants

In 1986, IBM introduced an 8 MHz upgrade for the AT, which boosted the clock speed of the processor from the original 6 MHz through a new Type 3 . This upgrade was released in April 1986 as part of the enhanced Models 319 and 339 configurations and maintained compatibility with existing AT chassis, allowing users to swap the without replacing the full system enclosure. Model 319 featured 512 KB base RAM and a 30 MB hard drive, while Model 339 added an enhanced 101-key keyboard. The higher clock speed improved overall performance for demanding applications, supported by updated dated November 15, 1985. These configurations offered 512 KB of base RAM on the , upgradable to 640 KB onboard, to better support multitasking and larger software loads under or PC DOS. They could be further expanded using IBM's official options. Regional variants included localized keyboards and power supplies adapted for standards like 220-240V AC, while retaining the core 80286 architecture and expansion capabilities. Accessory options expanded the AT's utility in enterprise environments, notably the AT/GX emulator card, which provided hardware-based compatibility for connecting to systems via the 3270 protocol. This card, compatible with the AT's ISA bus, enabled seamless integration as a for and terminal emulation without sacrificing general-purpose computing functions. The maximum factory configuration without third-party expansion boards reached 3 MB of RAM, achieved by combining onboard with up to two IBM 512 KB expansion modules installed in dedicated slots. Production of the Personal Computer AT began to phase out in 1987 with the introduction of the IBM Personal System/2 (PS/2) line, which aimed to succeed the AT through advanced features like Micro Channel Architecture. Remaining inventory and upgrade kits were sold through authorized channels until 1988, marking the end of official support for the 5170 model as IBM shifted focus to the PS/2 ecosystem.

Software Ecosystem

Operating Systems

The IBM Personal Computer AT (Model 5170) shipped with 3.00 as its bundled operating system, released in August 1984 specifically to support the AT's hardware advancements, including compatibility with the 1.2 MB high-density 5.25-inch floppy drives and hard disk partitioning using the FAT16 file system, allowing partitions up to 32 MB in size. This version of DOS built on prior releases by incorporating support for the AT's 80286 processor in , enabling efficient handling of larger storage media without requiring third-party drivers for basic operations. Beyond the standard DOS installation, the AT supported more advanced operating systems that leveraged its protected mode capabilities. IBM OS/2 1.0, jointly developed with Microsoft and announced in April 1987, was previewed and designed for 80286-based systems like the AT, providing a 16-bit multitasking environment with a command-line interface and support for up to 16 MB of memory in protected mode. Additionally, Microsoft's Xenix, a Unix-like operating system variant licensed from AT&T, was certified for the AT and utilized the 80286's protected mode to enable multi-user and multitasking features, making it one of the earliest Unix implementations on PC hardware. The AT's software bundle included as the primary BASIC interpreter, distributed on the DOS installation floppies rather than embedded in ROM, reflecting the removal of the cassette-based BASIC variant from earlier models due to the absence of a cassette port on the AT. For early multitasking on DOS, users could employ third-party solutions like Quarterdeck's DESQview, released in 1985, which provided text-mode windowing and task switching for DOS applications, or Microsoft's , also launched in 1985, offering with tiled windows atop DOS. Installation of PC DOS 3.00 on the AT typically began by booting from a set of 5.25-inch floppy disks, with the process involving formatting the hard drive (if present) and copying system files, after which the OS could access the AT's (RTC) chip via new DOS commands like DATE and TIME for automatic system date and time setting upon boot. This RTC support eliminated the need for manual date entry required on prior IBM PCs without battery-backed clocks. The AT maintained backward compatibility with software designed for the original PC and XT models running under DOS 2.x, allowing most existing applications to execute in without modification.

Compatibility and Backward Support

The IBM Personal Computer AT was engineered to ensure with the software ecosystem of the earlier IBM PC and PC XT models. The 80286 processor supported operation, which emulated the addressing and instruction set of the 8086/8088 processors, allowing all existing IBM PC and XT software to run without modification. This compatibility was essential for maintaining the large library of applications developed for the 8088-based systems, including and games. However, to leverage the 80286's capabilities—such as expanded memory addressing up to 16 MB and enhanced multitasking—software developers needed to recompile or rewrite programs specifically for the new architecture. On the hardware side, the AT's 16-bit ISA expansion bus provided bridging for peripherals from the 8-bit ISA bus of the PC XT. The bus design allowed 8-bit cards to function in the AT's slots through compatibility signaling, where the extra address and data lines were ignored, enabling users to reuse expansion cards like graphics adapters and network interfaces without replacement. For storage compatibility, the AT's controller supported the 360 KB double-density format used in XT systems, allowing direct integration of such drives with appropriate density settings. The AT's initial BIOS, stored in ROM and dated January 10, 1984, prioritized compatibility but had limitations, including the absence of functionality, which necessitated manual configuration of peripherals via jumpers and setup utilities. This BIOS structure supported real-mode operations for legacy hardware while introducing protected-mode routines for new features, though it required users to handle manually. Software such as Windows 1.0, released in 1985, took advantage of the AT's increased memory capacity—up to 3 MB via expansion—to deliver smoother multitasking and graphical interfaces compared to XT performance. To facilitate transitions from XT systems, IBM supplied an advanced diagnostic diskette with the AT, which included tests for verifying the functionality of XT peripherals when installed in the AT bus, such as serial ports, parallel adapters, and fixed-disk controllers. These diagnostics helped identify compatibility issues early, ensuring reliable integration of mixed-generation hardware. DOS 3.00, bundled with the AT, included enhancements like support for 1.2 MB floppy drives and larger hard disks to bridge the gap between XT and AT environments.

Operational Challenges

Hardware Reliability Issues

The IBM Personal Computer AT (model 5170) experienced several notable hardware reliability issues, particularly with its storage subsystems, which contributed to user frustration in the mid-1980s. The original 20 MB hard disk drive, manufactured by Computer Memories, Inc. (model 6426-S), suffered from exceptionally high failure rates, often necessitating multiple replacements per system due to mechanical instability and data integrity problems that led to frequent data loss. These drives failed to meet the AT's performance specifications for track-to-track and average seek times, exacerbating reliability concerns in production environments. In response, IBM conducted a supplier switch to Seagate's ST-4026 model and released firmware updates along with enhanced diagnostic tools in May 1985 to bolster error detection and correction mechanisms, aiming to mitigate data corruption risks. Upgrades involving Run Length Limited (RLL) controllers on 20 MB Seagate ST-225 drives, intended to expand capacity to approximately 30 MB, introduced additional vulnerabilities; early implementations often resulted in owing to inadequate error correction tailored for the higher encoding density on legacy (MFM) hardware. This compatibility-related drive issue compounded the AT's storage challenges, though it was less prevalent in stock configurations. For the floppy drive, the standard 1.2 MB high-density 5.25-inch mechanism encountered mechanical issues when handling unformatted double-density disks, occasionally causing the head carriage to jam due to mismatched rotational speeds (300 rpm for double-density vs. 360 rpm for high-density), which could require manual intervention or an external density selection switch on aftermarket controllers to resolve. Deteriorated lubrication in these drives over time also led to head carriage failures, necessitating cleaning and relubrication with silicone-based compounds for restoration.

Compatibility and Performance Limitations

The processor in the IBM Personal Computer AT supported , which enabled access to up to 16 MB of physical and provided features like and virtual addressing through segmentation and descriptor tables. However, most contemporary software, including applications, operated exclusively in to maintain compatibility with earlier 8086/8088-based systems, restricting effective memory access to 1 MB total, with only 640 KB available as for programs. To utilize memory beyond 1 MB, users relied on specialized drivers such as the Expanded Memory Specification (EMS) for page-swapped access or the later Extended Memory Specification (XMS), which required switching to via software like , but these solutions introduced complexity and were not natively supported by DOS. The AT's expansion bus, while upgraded to a 16-bit ISA design operating at 8 MHz, suffered from bottlenecks when mixing legacy 8-bit cards with 16-bit peripherals, as the system inserted additional wait states to synchronize access. Specifically, 8-bit card operations incurred 2 to 4 wait states per cycle, extending bus cycle times to 750–1000 ns, compared to 0–2 wait states and 250–500 ns for 16-bit cards, effectively reducing throughput for mixed configurations to the equivalent of 4–5 MHz for legacy peripherals. This latency arose from the bus's , where the 80286 paused during transfers to accommodate slower 8-bit devices, limiting overall I/O efficiency in systems retaining older expansion cards. Although the 80286's facilitated basic multitasking capabilities, early implementations like (1985) and Windows/286 primarily relied on in , with limited use of for , leading to high CPU utilization due to inefficient context switching and the absence of optimized drivers. Without preemptive scheduling or dedicated kernel support—features more fully realized in later systems like 1.0 (1987)—task switching imposed significant overhead, often consuming substantial processor cycles and reducing responsiveness in multi-application scenarios. In benchmark comparisons, the PC AT demonstrated 50–100% better than the PC XT in tasks, such as basic arithmetic and string operations, owing to the 80286's improved instruction execution at 6 MHz compared to the XT's 4.77 MHz 8088. For instance, relative measurements showed the AT achieving roughly 1.5–2 times the speed of the XT in standard computing workloads. However, by 1986, with the introduction of 80386-based systems like the at 16 MHz, the AT lagged significantly, offering only about 25–50% of the 386's performance due to the latter's 32-bit and higher clock speeds.

Market Reception and Competition

Initial Sales and Reviews

The IBM Personal Computer AT achieved strong initial sales following its September 1984 launch. An estimated 350,000 units were sold by the end of 1985, contributing to 's personal computer division revenue exceeding $4 billion in 1984. The system's appeal stemmed from its advanced 80286 processor and enhanced capabilities, positioning it as a high-end upgrade for professional users transitioning from earlier IBM PC models. Priced at a base of $3,995 for the model 68 (256 KB RAM, no hard drive, system unit only), with configurations including 256 KB RAM, a monochrome display adapter, keyboard, and 1.2 MB floppy drive reaching around $4,000–$5,000, the AT was seen as expensive and targeted primarily at businesses. By 1986, IBM had reduced prices on AT models to improve competitiveness amid the growing clone market. Reviews from leading publications underscored the AT's strengths in performance and versatility while acknowledging its cost barriers. In 1984 coverage, praised the 80286's improved performance over the 8088 and expandability via 16-bit ISA slots, positioning it as a benchmark for professional computing. Similarly, InfoWorld's 1984-1985 coverage highlighted the high price tag but praised its architecture, including capabilities and multitasking potential, which promised longevity for business applications. These assessments reflected the AT's role in elevating expectations for PC capabilities. Adoption was strong in corporate environments, accelerating the proliferation of productivity software like spreadsheets (e.g., ) and databases (e.g., dBASE III). This segment-driven success, amplified by hype from the prior year's announcement, solidified IBM's early dominance in enterprise computing before clone proliferation intensified.

Cloning and Industry Influence

The rapid success of the IBM Personal Computer AT spurred the development of compatible clones, beginning with systems like the AT&T PC 6300 in late 1984, followed by the 286 in 1985, which reverse-engineered the AT in under a year. This was followed by the Tandy 3000 in 1985, the first PC AT clone from a major manufacturer, featuring the fully 16-bit processor and designed for broad compatibility with AT software and peripherals. The AT's (ISA) bus, reverse-engineered by these early cloners, quickly emerged as a industry standard for expansion slots, enabling modular hardware additions without proprietary restrictions. The clone market expanded dramatically in the mid-1980s, capturing a substantial portion of through lower and equivalent ; by , non-IBM systems (including compatibles) accounted for over 60% of the PC market, up significantly from prior years. For instance, AT&T's PC 6300, an XT-compatible based on the , launched in October at $2,495 for a configuration with 128K RAM and dual floppy drives, significantly undercutting the IBM AT's base price of $3,995 (system unit only). This price advantage, combined with reliable compatibility via clean-room engineered from firms like —whose AT debuted in —drove , as clones used off-the-shelf components to match or exceed AT capabilities at reduced costs. IBM responded aggressively to the cloning threat, filing lawsuits in 1985 against several manufacturers for alleged of its , but these actions largely failed as courts upheld the legality of reverse-engineered, non-infringing alternatives that preserved functionality without copying code directly. This prompted IBM to attempt modifications in later models like the PS/2 to reduce clone compatibility, but third-party developers quickly adapted their ROMs to maintain seamless interoperability. The AT's profoundly shaped the industry, with compatible systems—built around the 80286 processor, ISA bus, and —influencing the majority of personal computers sold by the late 1980s, shifting the market toward open standards and away from proprietary favored by competitors like Apple. This commoditization accelerated innovation in peripherals and software, as the ISA bus's standardization fostered a vibrant of add-ons, ultimately eroding IBM's control while democratizing access to advanced computing.

Legacy and Evolution

Long-Term Impact

The IBM Personal Computer AT's introduction of the (ISA) bus in 1984 established a foundational expansion standard for personal computers, which influenced subsequent bus designs by enabling modular hardware additions and fostering compatibility across systems. This 16-bit bus architecture evolved into more advanced interfaces, with PCI emerging in the early as a higher-performance replacement and (PCIe) further advancing serial connectivity for modern peripherals by the mid-2000s. Meanwhile, the AT's use of the processor introduced key concepts like operation and hardware, which became integral to the x86 family. These innovations laid the groundwork for the x86 lineage, with billions of processors shipped worldwide powering desktops, laptops, servers, and embedded devices as of 2024. The AT symbolized a pivotal shift toward open markets in personal computing, as IBM's decision to use off-the-shelf components and publish technical specifications encouraged third-party and , democratizing access to PC technology beyond proprietary systems. This openness accelerated industry standardization and competition, transforming the PC from a niche product into a mass-market . In contemporary culture, the AT has inspired retro emulation projects, such as PCem and for cycle-accurate hardware simulation, and DOSBox-X for running DOS-era applications and games, preserving its software legacy for enthusiasts and historians. As of 2025, working IBM PC AT units hold collector value on platforms like , typically ranging from $200 to $500 depending on condition and completeness, reflecting interest among vintage hardware enthusiasts. These systems also serve educational purposes, illustrating computing history in museums, universities, and online courses focused on the evolution of personal technology. Economically, the AT contributed to the peak performance of IBM's Division, which generated approximately $4 billion in revenue by amid rapid market growth, before the division's challenges led to its sale to in 2005 for $1.75 billion.

Successors and Timeline

The (PS/2) line, launched in April , directly succeeded the PC AT by introducing enhanced architecture aimed at regaining market control through integrated features like VGA graphics, 3.5-inch floppy drives, and 72-pin memory modules. Prominent early models included the PS/2 Model 50 and Model 60, both utilizing 10 MHz processors and the proprietary 16-bit (MCA) bus, which supported faster 32-bit data transfer and basic plug-and-play capabilities but required new adapters incompatible with prior ISA slots. In September 1988, IBM introduced the PS/2 Model 30 286 as a budget-oriented AT replacement, featuring a 10 MHz 80286 processor, 2 MB of RAM, and a hybrid design retaining 16-bit ISA expansion slots for broader peripheral compatibility while incorporating PS/2 standards like MCA for select components. Production of the PC AT ceased in mid-1987. The shift to PS/2 encountered significant hurdles due to the MCA bus's proprietary nature and licensing fees, which hindered compatibility with the vast ecosystem of AT peripherals and third-party add-ons, thereby delaying PS/2 uptake and enabling ISA-based clone producers to capture greater market share. Official hardware support for the PC AT concluded around 1990. Throughout the , AT-compatible clones proliferated and dominated personal computing until the era began in 1993, extending the ISA platform's influence beyond IBM's proprietary efforts.

References

  1. https://bitsavers.trailing-edge.com/pdf/ibm/pc/at/1502494_PC_AT_Technical_Reference_Mar84.pdf
  2. http://www.minuszerodegrees.net/misc/IBM%20PC%20AT%205170%20announcement%20letter.txt
  3. https://www.pcjs.org/machines/pcx86/ibm/5170/
  4. https://sharktastica.co.uk/resources/docs/IBM_186-052_5170-319-339_86.pdf
  5. https://spectrum.ieee.org/how-the-ibm-pc-won-then-lost-the-personal-computer-market
  6. https://www.computerhope.com/jargon/a/at.htm
  7. https://www.kornferry.com/insights/briefings-magazine/issue-62/the-dilemma-of-lotus-1-2-3
  8. https://au.pcmag.com/old-desktop-pcs/42504/the-golden-age-of-ibm-pcs
  9. https://www.dollartimes.com/inflation/inflation.php?amount=4000&year=1984
  10. https://retrocmp.de/ibm/5170/ibm-al_184-097_1984-08-14.txt
  11. https://www.computinghistory.org.uk/det/6191/IBM-introduces-the-IBM-5170-PC-AT/
  12. https://www.pcjs.org/machines/pcx86/ibm/5170/ibm5170.php
  13. http://www.dosdays.co.uk/topics/get_a_286_running_like_a_386.php
  14. https://www2.iath.virginia.edu/elab/hfl0108.html
  15. https://apps.dtic.mil/sti/tr/pdf/ADA502229.pdf
  16. http://www.dosdays.co.uk/topics/pc_buses_explained.php
  17. https://pinoutguide.com/Slots/ISA_pinout.shtml
  18. https://www.minuszerodegrees.net/manuals/IBM_5170_Technical_Reference_1502243_MAR84.pdf
  19. https://gunkies.org/wiki/ST506_disk_interface
  20. https://www.motocraft.com/en/Article_detail/175/The-power-supply%27s-Original-IBM-PC%252C-XT-and-AT-standards
  21. https://bitsavers.org/pdf/ibm/pc/at/1502494_PC_AT_Technical_Reference_Mar84.pdf
  22. https://superuser.com/questions/1464592/should-an-old-ibm-pc-at-power-supply-only-function-while-loaded
  23. http://www.1000bit.it/ad/bro/Ibm/IBM-Hardware-Facts.pdf
  24. https://www.pingdom.com/blog/ps2-beginning-end-ibm-pc/
  25. https://winworldpc.com/product/pc-dos/3x
  26. https://www.pcjs.org/software/pcx86/sys/dos/ibm/3.00/
  27. https://www.os2museum.com/wp/dos/dos-3-0-3-2/
  28. https://www.os2museum.com/wp/os2-history/os2-1-0/
  29. https://www.os2museum.com/wp/ibm-pc-xenix/
  30. http://www.dosdays.co.uk/topics/basic.php
  31. https://www.dosdays.co.uk/topics/dos_versions_pt2.php
  32. https://cosmodoc.org/topics/ibm-pc/
  33. https://retrocomputing.stackexchange.com/questions/219/80286-can-switch-from-real-mode-to-protected-mode-but-why-not-back
  34. https://retrocomputing.stackexchange.com/questions/5165/choosing-a-vga-card-for-the-ibm-5162
  35. https://www.pcjs.org/machines/pcx86/ibm/5170/rom/
  36. https://www.pcjs.org/software/pcx86/diag/ibm/5170/2.07A/
  37. https://www.os2museum.com/wp/dos/dos-3-0-3-2
  38. https://dosdays.co.uk/computers/IBM%20PC-AT%20%285170%29/ibm5170.php
  39. https://dfarq.homeip.net/ibm-5170-pc-at/
  40. https://archive.org/stream/PC-Mag-1985-05-14/PC-Mag-1985-05-14_djvu.txt
  41. https://www.dosdays.co.uk/topics/hard_disks.php
  42. https://groups.google.com/g/comp.sys.ibm.pc/c/Vaoz6F2sLaY
  43. https://minuszerodegrees.net/5170/problems/5170_known_problems_issues.htm
  44. https://richard.downer.tech/2020/07/memory-in-the-ms-dos-era/
  45. https://retrocomputing.stackexchange.com/questions/8757/ibm-pc-expansion-card-latency
  46. https://www.filfre.net/2018/07/doing-windows-part-3-a-pair-of-strike-outs/
  47. https://www.latimes.com/archives/la-xpm-1986-11-10-fi-28852-story.html
  48. https://www.pcmag.com/news/the-golden-age-of-ibm-pcs
  49. https://www.cringely.com/2013/03/07/accidental-empires-chapter-9-clones/
  50. https://www.youtube.com/watch?v=OT3OPi6nHwM
  51. https://time.com/archive/6706612/cut-rate-computers-get-em-here/
  52. https://www.atarimagazines.com/creative/v11n6/32_ATampT_6300_clash_of_t.php
  53. https://www.allaboutcircuits.com/news/how-compaqs-clone-computers-skirted-ibms-patents-and-gave-rise-to-eisa/
  54. https://retrotechreads.substack.com/p/the-rise-and-fall-of-the-ibm-pc
  55. https://www.forbes.com/sites/timbajarin/2021/08/25/attack-of-the-clones-how-ibm-lost-control-of-the-pc-market/
  56. https://eureka.patsnap.com/article/from-isa-to-pcie-the-evolution-of-computer-expansion-buses
  57. https://book.huihoo.com/pc-architecture/chapter39.htm
  58. https://www.cpu-world.com/CPUs/80286/index.html
  59. https://thechipletter.substack.com/p/the-paradox-of-x86
  60. https://dosbox-x.com/
  61. https://kingofgng.com/eng/2025/11/04/a-flurry-of-updates-for-86box-dosbox-x-and-more-where-pc-emulation-is-heading/
  62. https://www.ebay.com/b/IBM-Vintage-Computers-and-Mainframes/162075/bn_1854644
  63. https://edtechmagazine.com/k12/article/2016/12/ibm-5150-began-classroom-technology-revolution
  64. https://news.lenovo.com/pressroom/press-releases/lenovo-completes-acquisition-ibms-personal-computing-division/
  65. https://www.ibm.com/history/ps-2
  66. https://www.computerhistory.org/timeline/1987/
  67. http://ps-2.kev009.com/pcpartnerinfo/ctstips/7d56.htm
  68. https://www.ardent-tool.com/qtechinfo/GJAN-43VS3S.html
  69. https://www.userlandia.com/home/ibm-ps2-model-30-286
  70. https://www.rs-online.com/designspark/the-pc-future-that-briefly-was-ibm-ps2-and-os2
  71. https://www.pcworld.com/article/465931/the_ibm_ps_2_25_years_of_pc_history.html
  72. https://www.os2museum.com/wp/
Add your contribution
Related Hubs
User Avatar
No comments yet.