IBM Personal Computer AT
| Company type | Personal computer model |
|---|---|
| Industry | Computer hardware |
| Predecessor | IBM PC XT |
| Founded | 1984 |
| Headquarters | Armonk, New York , United States |
| Products | IBM PC AT (model 5170) |
The IBM Personal Computer AT (model 5170), also referred to as IBM AT or PC/AT, debuted in 1984 as the fourth installment in IBM's IBM Personal Computer lineup. It succeeded the IBM PC XT along with its Portable PC version and was powered by the Intel 80286 microprocessor.
Name
While IBM never formally defined the abbreviation AT on the device or in official documentation, various sources and internal references suggest it stands for Advanced Technology.
History
The introduction of the AT in 1984 signaled a notable change in IBM’s approach, as the company usually waited for competitors to launch products before entering the market. Priced between $4,000 and $6,000, it delivered markedly enhanced performance with only a modest price increase compared to earlier slower IBM models. This pricing strategy caught competitors off guard, leaving them unable to offer similar systems at that year's COMDEX event in Las Vegas.
Features
As an IBM PC compatible system, the AT’s primary advancement was its upgrade from the 8088 to the more powerful 80286 processor. Like its predecessor, it optionally supported the Intel 80287 math co-processor to accelerate floating-point calculations.
A key innovation was the introduction of the AT bus, later renamed the ISA bus, which was a 16-bit expansion bus backward compatible with existing 8-bit cards. The bus architecture expanded available resources to fifteen IRQ lines and seven DMA channels—an increase from eight IRQs and four DMAs on the original PC—achieved by incorporating a second 8259A interrupt controller and an additional 8237A DMA controller. Certain IRQ and DMA channels were reserved exclusively for motherboard functions. Supporting components included the Intel 82284 Clock Driver and Ready Interface alongside the Intel 82288 Bus Controller.
The 80286 processor’s 24-bit address bus allowed memory addressing up to 16 MB of RAM.
PC DOS version 3.0 was released with support for new AT capabilities, such as early networking kernel functionality that was fully developed in subsequent releases.
The motherboard featured a battery-backed real-time clock (RTC) based on the Motorola MC146818 chip, which improved upon earlier PCs that required manual clock adjustments or separate expansion cards. This RTC included a 1024 Hz timer (IRQ 8), providing higher timing precision compared to the PC’s original 18 Hz timer.
In addition to timekeeping, the RTC contained 50 bytes of CMOS memory designated for BIOS configuration data. A disk-based BIOS setup utility stored system parameters here, replacing prior reliance on DIP switches found in earlier IBM PCs. Most AT-compatible clones implemented this setup program within ROM rather than requiring disk access.
Storage
The standard floppy drive was upgraded to a high-density 1.2 MB inch floppy disk drive, featuring 15 sectors of 512 bytes each, across 80 tracks on two sides—offering over triple the storage capacity of the previous 360 KB floppy disks used by PCs. However, compatibility issues arose when attempting to read older 360 KB disks (see Problems). Later AT models adopted inch floppy drives as well.
A default hard disk capacity of 20 MB was included. Early hard drives, produced by Computer Memories, were known for reliability problems.
Peripherals
The AT introduced a new keyboard design featuring an initial 84-key layout with a separate numeric keypad and additional LEDs indicating Caps Lock, Scroll Lock, and Num Lock statuses. It retained compatibility with the PC’s 5-pin DIN connector, but incorporated a bidirectional electrical interface enabling control over LED indicators, keyboard reset functions, and typematic rate adjustments. Subsequent AT models came equipped with extended keyboards such as the popular Model M keyboard featuring 101 keys.
A physical security lock was incorporated into the AT chassis to disable keyboard input and secure access to the system unit cover.
Display options included support for CGA, MDA, EGA, or PGA video cards.
The serial communication hardware saw an upgrade from the PC’s 8250 UART to the improved 16450, though both chips maintained single-byte buffers that limited performance during high-speed serial data transfers.
Models
| Model | IBM P/N | Launch Date | Discontinuation Date | Bus Type | Expansion Slots | Drive Bays | CPU | Clock Speed (MHz) |
Default RAM (KB) |
Max RAM (KB) |
FDD | HDD | Remarks | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| AT | 5170-068 | 1984-08-14 | 1987-06-30 | ISA, 16-bit | 8 | 3 | Intel 80286 | 6 | 256 KB | 512 KB | 1.2 MB | none | ||
| AT | 5170-099 | 1984-08-14 | 1987-06-30 | ISA, 16-bit | 8 | 3 | Intel 80286 | 6 | 512 KB | 512 KB | 1.2 MB | 20 MB | ||
| AT | 5170-239 | 1985-10-01 | 1986-09-02 | ISA, 16-bit | 8 | 3 | Intel 80286 | 6 | 512 KB | 512 KB | 1.2 MB | 30 MB | ||
| AT | 5170-839 | 1986-01-08 | 1987-06-30 | ISA, 16-bit | 8 | 3 | Intel 80286 | 6 | 512 KB | 512 KB | 1.2 MB | two 30 MB | Contains controllers for the IBM 4680 Store System | |
| AT | 5170-899 | 1986-01-08 | 1987-06-30 | ISA,16-bit | 8 | 3 | Intel80286 | "6" | "512 KB" | "512 KB" | "1.2 MB" | "two20 MB" | "Includes controllers for the IBM4680 Store System" | |
| "AT" | "5170-319" | "1986-04-02" | "1987-06-30" | "ISA",16-bit | "8" | "3" | "Intel80286" | "8" | "512 KB" | "512 KB" | "1.2 MB" | "30 MB" | ||
| "AT" | "5170-339" | "1986-04-02" | "1987-07" | "ISA",16-bit | "8" | "3" | "Intel80286" | "8" | "512 KB" | "512 KB" | "1.2 MB" | "30 MB" | ||
| "AT" | "5171-168" | "1986-10-21" | "1987-06-30" | "ISA",16-bit | "8" | "3" | "Intel80286" | "8" | "512 KB" | "512 KB" | "1.2 MB" | "none" | "Manufactured to TEMPEST standards" | |
| "AT" | "5171-339" | "1986-10-21" | "1987-07" | "ISA",16-bit | "8" | "3" | "Intel80286" | "8" | "512 KB" | "512 KB" | "1.2 MB" | "30 MB" | "Manufactured to TEMPEST standards" | |
| 'IBM Personal Computer AT/370' | '5170-599' | '1984-10-25' | '1987-04-02' | 'ISA',16-bit | '8' | '3' | 'Intel80286' | '6' | '512 KB' | '512 KB' | '1.2 MB' | '20 MB' | ||
| 'IBM Personal Computer AT/370' | '5170-739' | '1985-10-01' | '1987-04-02' | 'ISA',16-bit | '8' | '3' | 'Intel80286' | '6' | '512 KB' | '512 KB' | '1.2 MB' | '30 MB' | ||
| 'IBM Personal Computer AT/370' | '5170-919' | '1986-04-02' | '1987-04-02' | 'ISA',16-bit | '8' | '3' | 'Intel80286' | '6' | '512 KB' | '512 KB' | '1.2 MB' | '30 MB' | 'Includes an additional peripheral RAM of 1 MB total' | |
| 'IBM Personal Computer AT/370' | '5170-939' | '1986-04-02' | '1987-04-02' | 'ISA',16-bit | '8' | '3' | 'Intel80286' | '6' | '512 KB' | '512 KB' | '1.2 MB' | '30 MB' | 'Equipped with Enhanced Keyboard; includes peripheral RAM totaling 1 MB' |
Power supply
The IBM PC AT was equipped with a switching power supply rated at 192 watts, offering considerably higher output than the XT's standard 130-watt unit.
According to IBM's technical documentation, the AT's power supply needed a minimum current draw of 7.0 amperes on the +5 V rail and at least 2.5 amperes on the +12 V rail to function properly. Because the motherboard itself imposed minimal load on the +12 V line, base models lacking a hard disk incorporated a resistor rated at 5 ohms and dissipating 50 watts connected to the hard drive power connector’s +12 V line, which drew approximately 2.4 amperes and consumed about 28.8 watts to satisfy this requirement.
Problems
In addition to the instability of traditional hard disk drives, high-density floppy drives introduced their own complications. Certain AT models combined a single high-density (HD) drive with a double-density (DD) 360 KB drive. Notably, HD floppy disks were only compatible with HD drives.
The drives lacked the capability to automatically identify the disk type, with differentiation limited to an asterisk marking on the faceplate of the 360 KB drive. Occasionally, an HD disk could be used briefly in a DD drive; however, the DD write heads generated insufficient magnetization on the high-coercivity HD media, leading to frequent read failures.
On the other hand, HD drive heads featured tracks half as wide as those of DD drives and were unable to completely erase data written by DD drives. When DD disks were overwritten using an HD drive, they remained readable by HD drives but often caused errors in DD drives due to overlapping signals from residual and new data, resulting in corrupted reads on DD devices.
Clones
The architecture of the PC AT was essentially open in function, and IBM's efforts to trademark the AT designation largely did not succeed. Numerous 286-based personal computers were designed following this model and marketed as AT-compatible. This term also applied to systems utilizing the same power supply, chassis, and motherboard layout as the IBM 5170. The phrase AT-class described computers that supported identical BIOS features, employed 80286 or more advanced CPUs, included 16-bit expansion slots, used the same keyboard interface, and incorporated 1.2 MB inch floppy drives—core technical characteristics of the IBM PC AT.
New entrants into the PC compatible market quickly released AT-compatible machines, with sales estimated at approximately 100,000 units by late 1985. In the United States, notable AT clones included models such as the Tandy 3000, Compaq Deskpro 286, HP Vectra, Zenith Z-286, Epson Equity II+ and III models, and Commodore PC-30 and PC-40. European markets typically featured less prominently branded AT clones.
The AT bus evolved into the standard known as ISA (Industry Standard Architecture), while earlier PC XT slots were retrospectively classified as 8-bit ISA. IBM’s original Seagate ST506-compatible disk interface underwent enhancements and was standardized as ATA ("AT Attachment") through collaboration between Western Digital and Compaq in 1986; it was later renamed PATA (Parallel AT Attachment). Also called IDE (Integrated Drive Electronics), this interface integrated the controller directly within the drive rather than relying on a separate controller card.
Reception
By January 1985, demand for AT machines had surpassed IBM's and its suppliers' production capabilities, with total sales estimated at around 350,000 units by the end of that year.
In December 1984, Creative Computing lauded IBM for implementing a "refreshingly standard" keyboard design but remarked that "for most users, AT power is excessive ... a cutting-edge system at a premium price point." That same month, editor David H. Ahl recognized the AT as the top desktop computer "when cost is not an issue," describing it as "an innovative, state-of-the-art system leaving competitors struggling."
An industry expert writing for Computerworld in 1985 noted that the sophisticated features of the AT underscored IBM's strategic emphasis on personal computers rather than minicomputers.
Successor
IBM introduced its Personal System/2 (PS/2) series on April 2, 1987, branding it as IBM’s second generation of PC architecture. Initially, IBM planned to maintain production of some first-generation models including the AT. However, by June 1987, IBM declared full discontinuation of the PC/XT and announced intentions to cease PC/AT manufacturing by July. While the PC/XT was succeeded by the compatible PS/2 Model 30, no direct replacement was offered for the AT. Buyers had to decide between adopting IBM’s proprietary Micro Channel architecture, which rendered existing 16-bit ISA expansion cards incompatible, or purchasing third-party clone systems to preserve compatibility. In September 1988, IBM released the PS/2 Model 30 286, equipped with an Intel 80286 processor and retaining support for 16-bit ISA slots—providing a genuine upgrade path for existing AT users.
See also
References
- Notes
- IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
- PC AT entry at old-computers.com