KB10011: IBM PC Models Compared – 5150 to PS/2

IBM produced its own line of personal computers from 1981 through the 1990s. While the "IBM PC compatible" market eventually became dominated by third-party manufacturers, IBM's own models set the standards that defined the platform. This article covers every major IBM-branded PC model, the bus architectures they introduced, and what distinguishes each from the others.

IBM PC (Model 5150, 1981)

The machine that defined the standard. The original IBM Personal Computer used an Intel 8088 processor running at 4.77 MHz. Despite having a 16-bit internal architecture, the 8088 used an 8-bit external data bus, which kept component costs down. The base configuration shipped with 16 KB of RAM (expandable to 256 KB on the motherboard), a cassette port for storage, and IBM's CGA (Color Graphics Adapter) or MDA (Monochrome Display Adapter). It had five 8-bit ISA expansion slots and a 63-watt power supply. An optional Disk Operating System (PC DOS 1.0, developed by Microsoft) was available alongside 5.25-inch floppy drives. The open architecture with published technical specifications and use of off-the-shelf components was a deliberate design choice that enabled the entire clone industry.

IBM PC/XT (Model 5160, 1983)

The XT ("eXtended Technology") was an evolutionary upgrade that made the PC more practical for business use. It kept the same 8088 at 4.77 MHz but shipped with 128 KB of RAM standard (expandable to 640 KB), increased the expansion slots from five to eight, and—most importantly—included a 10 MB Seagate ST-412 hard drive as standard. The power supply was upgraded to 130 watts to handle the hard drive. IBM dropped the cassette port since floppy and hard drives had made cassette storage obsolete. The XT established the expectation that a business PC should include a hard drive.

IBM PCjr (Model 4860, 1984)

The PCjr ("PC Junior") was IBM's ill-fated attempt at the home market. It used the same 8088 at 4.77 MHz with 64 KB of RAM (expandable to 128 KB), but took a different design approach. It featured two ROM cartridge slots (up to 64 KB each), an infrared wireless keyboard (initially with a widely criticized chiclet design, later replaced with a full-travel keyboard), built-in joystick ports, and a three-voice sound chip that was more capable than the PC's simple beeper. Graphics exceeded the standard CGA with additional modes. However, performance suffered because the video hardware shared access to system RAM, inserting wait states that slowed the CPU. The PCjr used "sidecars" for expansion instead of ISA slots, was incompatible with many PC programs due to its memory architecture, and was priced too high for the home market. IBM discontinued it in 1985 after poor sales. Despite its commercial failure, the PCjr's enhanced graphics and sound modes were adopted by the Tandy 1000, which became far more successful.

IBM Portable PC (Model 5155, 1984)

The Portable PC was essentially an XT motherboard in a luggable case with a built-in 9-inch amber CRT monitor. It weighed approximately 30 pounds and used the same 8088 at 4.77 MHz with 256 KB of RAM. It had two 5.25-inch floppy drives (no hard drive) and retained the XT's ISA expansion capability with somewhat fewer slots. It was fully compatible with standard PC software but was heavy and expensive compared to the Compaq Portable, which had beaten it to market.

IBM PC/AT (Model 5170, 1984)

The AT ("Advanced Technology") was a major generational leap. It introduced the Intel 80286 processor, initially at 6 MHz, later upgraded to 8 MHz in the Model 339. The 286 was a true 16-bit processor with a 16-bit data bus, running roughly three to four times faster than the 8088 in real-world applications. The AT also introduced the 16-bit ISA bus (extending the original 8-bit bus with an additional connector), which became the standard expansion bus for the next decade. Other firsts included the high-density 1.2 MB 5.25-inch floppy drive, a 20 or 30 MB hard drive, a real-time clock with battery backup, and 256 KB to 512 KB of RAM standard (expandable to 16 MB via the 286's protected mode, though DOS software could not use it directly). The AT keyboard connector and layout also became standard. RAM configurations varied by model number: Models 68 and 99 (1984) shipped with 256 KB; Model 239 (1985) had 512 KB standard.

IBM Convertible (Model 5140, 1986)

IBM's first laptop-style computer, the Convertible used an 80C88 processor (the CMOS variant of the 8088) at 4.77 MHz with 256 KB of RAM. It had a built-in LCD screen, two 3.5-inch 720 KB floppy drives (IBM's first use of the 3.5-inch format), and a detachable screen. Despite the laptop form factor, it was heavy, the LCD was mediocre, and the 8088-class performance was outdated by 1986. It was not commercially successful.

IBM PS/2 (1987–1990s)

The PS/2 (Personal System/2) was IBM's ambitious attempt to reclaim control of the PC platform from the clone makers. It introduced several important standards alongside the controversial MCA bus.

PS/2 Model 25 and Model 30

Entry-level machines using the 8086 (Model 25) or 8086/80286 (Model 30) with ISA slots rather than MCA. The Model 25 was an all-in-one with a built-in monochrome or color monitor. These were low-end machines intended for price-sensitive buyers and education.

PS/2 Model 50 and Model 60

Mid-range systems using the 80286 at 10 MHz with the new Micro Channel Architecture (MCA) bus. MCA was a 32-bit bus with bus mastering and automatic configuration (no jumpers), but it was proprietary, incompatible with ISA cards, and required license fees from IBM. This was a deliberate move to cut out clone manufacturers, but it backfired—the industry rallied around the competing EISA standard instead.

PS/2 Model 70 and Model 80

High-end systems using the Intel 80386 processor at 16 to 25 MHz with the MCA bus. The Model 80 was a floor-standing tower. These were powerful machines for their era but the MCA lock-in limited their appeal in a market that had come to expect open standards.

Lasting PS/2 Contributions

While MCA failed to displace ISA, the PS/2 line introduced several standards that lasted for decades: the PS/2 mini-DIN keyboard and mouse connectors (still found on some motherboards today), the VGA (Video Graphics Array) graphics standard with 640×480 resolution and 256 colors from a 262,144-color palette, and the 3.5-inch 1.44 MB floppy drive as the standard floppy format.

Quick Comparison Table

Model	Year	CPU	Speed	Bus	Base RAM	Notable Feature
PC 5150	1981	8088	4.77 MHz	8-bit ISA	16 KB	The original IBM PC
XT 5160	1983	8088	4.77 MHz	8-bit ISA	128 KB	First with hard drive standard
PCjr 4860	1984	8088	4.77 MHz	Sidecar	64 KB	Home market, enhanced sound/graphics
Portable 5155	1984	8088	4.77 MHz	8-bit ISA	256 KB	Luggable with built-in CRT
AT 5170	1984	80286	6–8 MHz	16-bit ISA	256–512 KB	16-bit bus, 1.2 MB floppy
Convertible 5140	1986	80C88	4.77 MHz	Proprietary	256 KB	IBM's first laptop
PS/2 Model 30	1987	8086	8 MHz	8-bit ISA	640 KB	Entry-level PS/2
PS/2 Model 50	1987	80286	10 MHz	MCA	1 MB	First MCA model
PS/2 Model 80	1987	80386	16–25 MHz	MCA	1–2 MB	High-end 386, tower case

Collector and Restoration Notes

The 5150 and 5160 are generally reliable machines, but their power supplies can develop issues after decades. The tantalum capacitors on 5150 motherboards are a known failure point. The PC/AT's Dallas real-time clock chip (a potted module containing a battery) will eventually die, losing CMOS settings on every power cycle; aftermarket replacements are available. For PS/2 MCA machines, finding expansion cards can be challenging since MCA was never widely adopted outside IBM. VGA monitors from the PS/2 era are well-suited for use with earlier machines via appropriate adapters. On all models, check for corrosion from leaked CMOS batteries and inspect floppy drives for worn belts and dirty heads.