IBM PS/2

IBM Personal System/2 (PS/2)
Company type	Product line
Industry	Computer hardware
Predecessor	IBM Personal Computer, XT, AT, PC Convertible
Founded	1987
Founder	IBM
Defunct	July 1995
Fate	Phased out
Headquarters	Armonk, United States
Area served	Worldwide
Products	Personal computers, computer peripherals, Micro Channel architecture, VGA standard, PS/2 port
Owner	IBM

The Personal System/2 (PS/2) represents IBM's second generation of personal computers. Introduced in 1987, it succeeded the IBM Personal Computer, XT, AT, and PC Convertible series within IBM's product range. Numerous innovations from the PS/2—such as the 16550 UART serial interface, the 1440 KB 3.5-inch floppy disk format, 72-pin SIMM memory modules, the PS/2 port, and the VGA video standard—eventually became widely adopted industry benchmarks.

The PS/2 family sought to reclaim market dominance by unveiling the proprietary Micro Channel architecture (MCA) on its premium models. Despite MCA’s technical advancements, its lack of compatibility with existing IBM PC-compatible devices and IBM’s licensing model, which imposed royalties per system, discouraged many hardware manufacturers. Alongside the PS/2 launch, IBM introduced the OS/2 operating system as the preferred platform for systems powered by Intel 80286 or newer processors; initially, only IBM PC DOS 3.3 was available, with OS/2 1.0 and Microsoft Windows 2.0 arriving several months later. Additionally, IBM provided AIX PS/2—a UNIX variant—targeted at PS/2 models equipped with Intel 386 or later CPUs.

The PS/2 initially found favor among business clients, with sales reaching three million units within eighteen months by September 1988. Nonetheless, its performance in the consumer sector was hindered by a weak connection between MCA technology and the nascent OS/2 platform, which failed to justify its higher price compared to standard hardware running Microsoft Windows on competing IBM PC compatibles. In response, rival companies developed the EISA bus standard as an alternative to MCA. By 1992, IBM had significantly diminished influence over market standards through its proprietary technology. The PS/2 series was officially phased out in July 1995.

Technology

Predecessors to the PS/2
Model	Release Year
IBM Personal Computer	1981
IBM Personal Computer XT	1983
IBM Portable Personal Computer	1984
IBM PCjr	1984
IBM Personal Computer/AT	1984
IBM PC Convertible	1986
IBM Personal Computer XT 286	1986

While maintaining software compatibility with the foundational PC/AT/XT systems that supported the PC clone ecosystem, IBM’s PS/2 introduced distinct hardware features. It incorporated two BIOS variants: ABIOS (Advanced BIOS), designed for a new protected mode interface utilized by OS/2, and CBIOS (Compatible BIOS), which ensured full backward compatibility with PC/XT/AT software—including Cassette BASIC support. Although IBM did not disclose BIOS source code publicly, it pledged to document BIOS entry points.

Micro Channel architecture

Some PS/2 configurations featured the Micro Channel Architecture (MCA), inspired conceptually by the channel design of IBM System/360 mainframes. MCA provided technical improvements over Industry Standard Architecture (ISA), facilitating faster internal data transfers comparable to those later achieved by PCI standards. It supported one-to-one, card-to-card, and multiple simultaneous card transactions—a capability that anticipated features found in PCI-X.

MCA incorporated bus mastering, arbitration mechanisms, and an early iteration of plug-and-play hardware management. However, system setup required a dedicated configuration disk unique to each computer and expansion card combination; this disk automated interrupt assignments and settings adjustments while saving updates back to itself. Although this process lowered maintenance expenses for small businesses by simplifying configuration tasks, it posed logistical difficulties for large organizations managing numerous PCs due to the necessity of maintaining individual setup disks for each machine. Consequently, MCA adoption remained largely confined to PS/2 systems alone.

Layout

The PS/2 IBM Model M keyboard preserved the 101-key configuration found in the earlier IBM PC/AT Extended keyboard. In European variants, a 102nd key was added immediately to the right of the left Shift key.

Interface

PS/2 introduced updated interfaces for keyboards and mice, commonly referred to as "PS/2" ports, which replaced the older 5-pin DIN connector with a more compact 6-pin mini-DIN design. Although largely replaced by USB peripherals in modern systems, these interfaces significantly influenced peripheral connectivity for many years. The PS/2 keyboard port shared electrical characteristics with the AT interface but featured a smaller connector; similarly, the mouse port employed a synchronous serial communication method.

The first desktop models, such as the Model 50 and Model 60, pioneered a cable-free internal architecture that utilized interposer circuit boards to link drives directly to the motherboard, facilitating tool-less disassembly and maintenance.

Additionally, PS/2 introduced the Extended BIOS Data Area (EBDA), which expanded buffer capacity primarily to support the dedicated mouse port, necessitating modifications to the traditional BIOS Data Area (BDA).

An important advancement was the implementation of bidirectional parallel ports, allowing for high-speed data transfers beyond conventional printer functions. This enabled support for devices like scanners and CD-ROM drives and enhanced printer communication efficiency.

Graphics

The majority of early PS/2 computers incorporated the new Video Graphics Array (VGA) frame buffer, superseding the older EGA standard. VGA increased video memory to 256 KB and supported resolutions of 640×480 with 16 colors and 320×200 with 256 colors, offering a palette of 262,144 colors compared to EGA’s 64.

The PS/2 series also introduced display standards such as the IBM 8514 and later the XGA.

Notable monitors and their peak resolutions included:

8504: 12-inch monochrome at 640×480
8507: 19-inch monochrome at 1024×768
8511/8512/8513: 12- or 14-inch at 640×480
8514: 16-inch at 1024×768
8515/8516: 14-inch at 1024×768
8518: 14-inch at 640×480
9515: 14-inch at 1024×768
9517: 16-inch at 1280×1024
9518: 14-inch at 640×480
38F4737: exclusive to P70/P75 models, featuring a 10-inch amber monochrome plasma screen

XGA monitors were multimode devices functioning as expansion cards that transparently passed VGA signals when operating outside high-resolution modes. Most displays listed supported standard VGA modes alongside higher resolution settings. However, certain workstation-targeted models (95xx series) operated exclusively at fixed refresh rates exceeding VGA specifications.

While these adapters did not become universal standards like VGA, they established the widely recognized resolution of 1024×768 associated with XGA. Entry-level systems such as Models 25 and 30 utilized a simplified VGA variant known as MCGA, which supported CGA modes and limited VGA resolutions but lacked EGA compatibility and did not support color at 640×480 resolution.

VGA Video Connector

All PS/2 graphics subsystems—including MCGA, VGA, 8514, and XGA—employed a 15-pin D-sub connector transmitting analog RGB signals. This replaced earlier digital color signaling methods restricted to fixed palettes. The analog signal approach provided enhanced color depth and compatibility with grayscale monitors. Unlike previous adapters such as MDA or Hercules, the VGA connector was transparent to software, allowing applications to run unaltered across different display types. However, software could not automatically distinguish between color and monochrome monitors, requiring users to manually select optimal display settings. Grayscale displays were cost-effective options often bundled with entry-level PS/2 units.

The VGA connector went on to become the dominant industry standard for monitors and projectors across both PC and non-PC platforms during the early 1990s, supplanting numerous older connector types.

Storage

IBM established the 3.5-inch floppy disk as the industry standard with the introduction of the PS/2 series, building upon Apple's earlier success with the Macintosh platform. This decision accelerated widespread adoption but reduced native support for 5.25-inch drives within IBM systems, complicating the integration of CD-ROM drives and limiting access to older IBM-compatible software. To accommodate data transfer requirements, IBM provided optional external 5.25-inch drives equipped with internal adapters for early PS/2 models.

At launch, IBM utilized 720 KB double density (DD) floppy drives on 8086-based systems and 1440 KB high density (HD) drives on models powered by the 80286 processor and above; subsequent PS/2 versions standardized on a 2880 KB capacity.

Unlike many clone systems, PS/2 floppy drives did not feature automatic capacity detection. IBM omitted hole detection mechanisms that differentiate between 720 KB and 1440 KB disks, enabling users to format a 720 KB disk as 1440 KB on a PS/2 machine; however, such disks remained readable only on PS/2 hardware.

The PS/2 line predominantly employed Mitsubishi floppy drives that combined power and data transmission through a single cable. Over time, these drives suffered from decreased reliability due to capacitor degradation.

Regarding internal hard disk storage, early PS/2 units integrated MFM or ESDI hard drives; some desktop variants utilized combined power and data cables similar to those used for floppy drives. Later iterations shifted to DBA ESDI or Parallel SCSI interfaces. Typically, desktop PS/2 models supported only one internal hard drive, with additional storage options available via external SCSI attachments.

Memory

The later PS/2 models introduced the 72-pin SIMM, which emerged as the prevailing RAM module standard by the mid-1990s in mid-to-late 486 and most Pentium desktop systems. These modules were 32 or 36 bits wide, supplanting the older 30-pin SIMM format that operated at 8 or 9 bits. The previous generation of SIMMs required installation in matched sets to align with CPU data bus widths, making them unsuitable for Pentium processors featuring 64-bit memory buses. The newer 72-pin SIMMs offered enhanced capacities ranging from 1 MB up to 128 MB and more flexible sizing options based on powers of two.

Several PS/2 models incorporated proprietary IBM SIMMs incompatible with standard modules; however, it was possible to adapt common SIMMs for use in PS/2 machines by rewiring their detection contacts.

Models

The initial PS/2 lineup comprised the Model 30, Model 50, Model 60, and Model 80, with the Model 25 arriving shortly thereafter.

Designed as entry-level replacements for the IBM PC and XT, Models 25 and 30 featured either an 8086 CPU running at 8 MHz paired with 512 KB RAM and 720 KB floppy drives or an upgraded 80286 processor. The 8086 variants included ISA expansion slots and integrated MCGA monitors, while the 80286 versions came with VGA displays. A compact Model M keyboard lacking a numeric keypad was standard, with an option for a full-sized keyboard. A rare later release, the PS/2 Model 25-SX, incorporated a 16 or 20 MHz 386 CPU, supported up to 12 MB of memory, included an IDE hard drive, VGA monitor, and featured 16-bit ISA slots—representing the highest specification within the Model 25 series (8525-L41).

The Model 30 was offered with either an 8086 or a 286 processor, equipped with a full-sized 101-key keyboard, standalone monitor, and three available 8-bit ISA expansion slots. Floppy drive configurations ranged from 720 KB in the older CPU models to 1440 KB on those with the higher-end processor. Both Models could be fitted with an optional ST-506 hard disk of approximately 20 MB; notably, in the Model 25 this hard drive replaced the second floppy drive using a proprietary form factor. The Model 30 supported up to four megabytes of RAM.

Subsequent ISA-based releases included the Model 30-286 (featuring an Intel 286 CPU), alongside Model 35 (IBM 8535) and Model 40 (IBM 8540), which incorporated Intel386SX or IBM386SLC processors.

Higher-tier models numbered above fifty utilized the Micro Channel Architecture bus and were predominantly equipped with ESDI or SCSI hard disks (with exceptions such as models60-041 and80-041 employing MFM drives). Models 50 (IBM8550) and 60 (IBM8560) operated on Intel286 CPUs; Models70 (IBM8570) and 80 were powered by Intel386DX processors; meanwhile, mid-range Model55 SX (IBM8555–081) utilized a386SX chip. The Model50 was upgraded to Model50 Z, featuring zero wait state memory and ESDI drives. Later revisions of Model70 and 80 incorporated Intel486 processors within IBM's Power Platform architecture.

Top-tier Models90 (IBM8590/9590) and 95 (IBM8595/9595/9595A) employed Processor Complex daughterboards that integrated CPU cores, memory controllers, MCA interfaces, and other system components. Processor Complex configurations ranged from a20 MHz Intel486 up to fully interchangeable units running at90 MHz Pentium speeds. The motherboard design was shared by the IBM PC Server500.

Additional Micro Channel-based PS/2 variants included the 65SX, featuring a16 MHz386SX; various models such as 53, 56, and 57 equipped with processors from386SXto486SLC2; Models76and77 incorporating486SXand486DX2 CPUs; as well as the486-based Model85.

The IBM PS/2E (IBM9533) represented IBM's inaugural Energy Star-compliant personal computer. It housed a50 MHz IBM486SLC processor, utilized an ISA bus architecture, featured four PC Card slots alongside an IDE hard drive interface, employed recycled plastics for environmental sustainability, consumed low power levels, and borrowed components from the ThinkPad laptop series.

The PS/2 Server195and295(IBM8600) were486-based dual-bus MCA servers supporting asymmetric multiprocessing capabilities; these systems were developed by Parallan Computer Inc.

Although not officially categorized within the PS/2 family, the IBM PC Server720 (IBM8642) stood as IBM's largest MCA-based server platform. It accommodated up to six Intel Pentium processors interconnected via Corollary C-bus technology and supported as many as eighteen SCSI hard disk drives while offering seven hybrid MCA/PCI expansion slots.

PS/2 Portable Computers, Laptops, and Notebooks

IBM developed a variety of portable and laptop PS/2 systems, including the Model L40 which utilized an ISA-bus 386SX processor, as well as the N33 (IBM's inaugural notebook-style computer released in 1991, Model 8533 with a 386SX CPU), the N51 (equipped with a 386SX/SLC), the P70 (featuring a 386DX), and the P75 (powered by a 486DX2).

The IBM ThinkPad 700C, sometimes referred to as "700C PS/2," incorporated MCA technology alongside a 486SLC processor.

6152 Academic System

Introduced in February 1988, the 6152 Academic System was designed by IBM's Academic Information Systems group specifically for higher education institutions. This workstation was built upon the PS/2 Model 60 framework but distinguished itself by incorporating a RISC Adapter Card on the Micro Channel bus. This co-processor allowed the system to execute ROMP-based software compiled for IBM's Academic Operating System (AOS), a specialized BSD UNIX variant available only to select universities.

The RISC Adapter Card contained the ROMP-C microprocessor—an advanced version of the ROMP used in IBM RT PC workstations—along with a memory management unit capable of virtual memory support, a floating-point coprocessor, and up to 8 MB of dedicated RAM. The 6152 marked the debut of the ROMP-C processor, which was later integrated into subsequent RT PC models.

Marketing

In the 1980s, IBM’s promotional efforts for its original PC and other hardware frequently featured imagery inspired by Charlie Chaplin.

Another notable advertising campaign cast actors from the television series M*A*S*H as members of a modern corporate workforce, echoing their iconic characters. Original participants included Harry Morgan, Larry Linville, William Christopher, Wayne Rogers, Gary Burghoff, Jamie Farr, and Loretta Swit; Alan Alda joined the campaign at a later stage.

Due to these campaigns achieving only modest impact, IBM eventually severed ties with its global advertising agencies—managing accounts reportedly valued at over $500 million annually—resulting in one of the largest account reviews ever conducted in corporate history.

Sales

Despite efforts with PC-based Models 30 and 25, the PS/2 series largely failed to capture consumer market share. IBM’s aggressive promotion of its proprietary MicroChannel architecture combined with the premature OS/2 1.x operating system provoked resistance within the industry, prompting alternative standards such as VESA, EISA, and PCI to emerge. The more robust OS/2 version 2.0 did not arrive until 1992, missing critical early adoption windows.

Throughout the late 1980s, IBM endured considerable financial setbacks, losing its position as market leader and eventually yielding top PC manufacturer status first to Compaq and subsequently Dell. Prior to PS/2’s introduction, Boca Raton employed around 10,000 staff members; within seven years, IBM faced $600 million in unsold stock alongside widespread layoffs. Following PS/2’s failure to establish a new industry standard, IBM reverted to producing ISA-based systems through its entry-level PS/1 line and later introduced more compatible product families such as Aptiva and PS/ValuePoint.

Nevertheless, PS/2 models found traction within enterprise environments where their dependability, straightforward maintenance, and IBM’s robust support justified premium pricing. Mid-tier models like the 55SX and 56SX consistently ranked among best-sellers during their market tenure. Production of later PS/2 units continued into the late 1990s until IBM eventually divested this business segment.

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