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IBM PC (5150)

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IBM Personal Computer
IBM 5150 Personal Computer with 5151 monochrome display and Model F keyboard
Specifications
DeveloperIBM Entry Systems Division, Boca Raton (Don Estridge)
ManufacturerIBM
TypeDesktop personal computer
ReleasedAugust 12, 1981
DiscontinuedApril 2, 1987
Intro priceUS$1,565 (base, 16 KB, no FDD); typical system ~US$3,000
Units sold~3 million (1981-87 line)
CPUIntel 8088 @ 4.77 MHz
Optional Intel 8087 FPU
MemoryEarly board: 16-64 KB on-board (4116 DRAM)
Later board: 64-256 KB on-board (4164 DRAM)
Expandable to 640 KB via ISA RAM cards
StorageCassette tape (built-in port); one or two 5.25" floppy drives (single-sided 160 KB, then double-sided 320/360 KB)
DisplayMDA + IBM 5151 monochrome display (80×25 text), or CGA + IBM 5153 colour display (640×200 mono, 320×200 4-colour)
SoundInternal PC speaker, driven by Intel 8253-5 PIT channel 2
Dimensions19.6" W × 16" D × 5.5" H (498 × 406 × 140 mm)
Weight~21 lb (9.5 kg) base unit only
OS / FirmwareIBM PC DOS 1.0 (1981) through PC DOS 3.3 (1987); CP/M-86; UCSD p-System; Cassette BASIC in ROM
PredecessorIBM 5100 (in spirit); IBM Datamaster (immediate ancestor)
SuccessorIBM PC XT (5160)
CodenameProject Chess / Acorn
Model no.5150

The IBM Personal Computer (model 5150) is the original IBM PC, announced on August 12, 1981 and shipped from September 1981 through April 2, 1987. It was developed in roughly one year by a twelve-person team in Boca Raton, Florida under Don Estridge and William C. Lowe, using off-the-shelf parts to bring an IBM-branded microcomputer to market quickly. The architecture it established — Intel 8088 CPU, 8-bit ISA expansion bus, MS-DOS, an open BIOS interface, and Microsoft Disk BASIC in ROM — became the de-facto standard for personal computers for the following two decades.

Architecture and Processor

The 5150 uses an Intel 8088 running at 4.77 MHz, a 16-bit CPU with an 8-bit external data bus chosen to reduce board cost. A second socket on the motherboard accepts an Intel 8087 floating-point coprocessor, sold separately.

The CPU is supported by IBM's chosen 8088 'family' chipset:

  • Intel 8259A — programmable interrupt controller (8 IRQ lines).
  • Intel 8237A-5 — DMA controller (4 channels).
  • Intel 8253-5 — programmable interval timer (3 channels: system tick, DRAM refresh, speaker).
  • Intel 8255A-5 — programmable peripheral interface (keyboard input, DIP-switch readback, speaker enable).
  • Motorola MC6845 — CRT controller (on the MDA and CGA display adapters, not on the motherboard).

Memory and ROM

Two motherboard revisions exist, distinguished by the amount of on-board RAM they accept.

IBM 5150 Motherboard Revisions
Revision On-board RAM RAM type Banks Years sold
16KB-64KB (early) 16 KB – 64 KB 4116 (16 Kbit, 16-pin) 4 × 16 KB August 1981 – ~early 1983
64KB-256KB (later) 64 KB – 256 KB 4164 (64 Kbit, 16-pin) 4 × 64 KB ~early 1983 – April 1987

The motherboard carries 40 KB of ROM in five sockets:

  • U33 — 8 KB BIOS ROM.
  • U29, U30, U31, U32 — 32 KB of Cassette BASIC (Microsoft BASIC C1.0/C1.1).

There are four BIOS revisions, all dated rather than numbered (date format MM/DD/YY):

IBM 5150 BIOS Revisions
Date IBM part number Compatible board Notes
04/24/81 5700051 16KB-64KB only Recognises only 544 KB total RAM; ignores BIOS expansion ROMs (no EGA/VGA, no HDD)
10/19/81 5700671 16KB-64KB only Same two limitations as 04/24/81
08/16/82 5000024 (rare/unreleased) Has a bug where EGA / 5150-compatible VGA fail if SW1 video bits are ON-ON
10/27/82 1501476 16KB-64KB or 64KB-256KB Removes the 1981 BIOS limits; requires all four motherboard banks populated; "C800 ROM" corruption beeps 1L+2S as if video failure

System configuration is set by two banks of DIP switches (SW1 and SW2) on the motherboard — there is no battery-backed CMOS on the 5150.

Storage and Expansion

  • Cassette port (DIN-5, rear panel) — for tape data, paired with the Cassette BASIC ROM. Essentially unused once floppy drives became standard.
  • Floppy disk drive — one or two half-height 5.25" drives, originally Tandon TM100-1 single-sided (160 KB), later Tandon TM100-2 / TM100-2A or Micropolis 1015-5 double-sided (320 KB at PC DOS 1.1, 360 KB at PC DOS 2.0+). The drives connect to an IBM FDD Adapter ISA card — the motherboard has no on-board floppy controller.
  • No internal hard drive support from the original PSU (63.5 W). The later Type 2 (130 W) PSU made it possible to fit a 10 MB or 20 MB MFM hard drive on an IBM Fixed Disk Adapter.
  • 5 × 8-bit ISA slots, spaced 1" apart (the 5150's slot spacing is wider than the later 5160 XT). Typical slot allocation: video card, FDD adapter, RAM expansion, serial card, modem or printer card.

Display

The 5150 has no on-board video. All video is provided by an ISA adapter card driving a separate monitor:

Keyboard

The 5150 ships with the IBM Model F (83-key) keyboard — the original capacitive buckling-spring keyboard. It connects to the rear of the case via a 5-pin DIN connector and a curly coiled cord. The Model F preceded the more familiar Model M and uses an 83-key XT-style layout: function keys F1-F10 on the left, no separate cursor cluster (the keypad doubles as cursor keys), and no Windows or context keys. Unlike later AT-class machines, the 5150 has no keyboard controller IC on the motherboard — the keyboard interface is a simple shift-register feed read by the 8255 PPI.

Power Supply

Two PSU revisions exist:

  • Type 1 — 63.5 W, sufficient for the base configuration of motherboard + floppy drive(s).
  • Type 2 — 130 W, introduced to support an internal hard drive. Externally identical.

Both produce +5 V, +12 V, −5 V, −12 V on the motherboard's P8/P9 connectors. The −5 V rail is required by both the 4116 DRAM (early board) and the cassette circuitry (chip U1 on the later board); a modern ATX adapter cannot drive this rail and causes specific failure modes (see Troubleshooting).

General Maintenance

Cleaning procedures, capacitor inspection, PSU voltage checks, and preventive care for the 5150 are documented in IBM PC (5150) Maintenance Guide.

Capacitor Replacement Guide

The 5150 uses a mix of through-hole electrolytic capacitors on the PSU and tantalum capacitors on the motherboard and ISA cards. Short-circuit tantalum failure is one of the most common faults on this machine. Procedures and the per-board capacitor list are documented in IBM PC (5150) Capacitor Replacement Guide.

Troubleshooting

POST beep codes, parity errors, RAM bank 0 failure, keyboard 301 errors, and PSU diagnostics are covered in IBM PC (5150) Troubleshooting Guide.

Technical Documentation

  • The original IBM 5150 Technical Reference (part 6025005, August 1981) gives the full schematics, BIOS listing, and DIP-switch reference.
  • See IBM Personal Computer Hardware Reference Library for the broader manual set.
  • For all four BIOS dumps and per-revision differences, see minuszerodegrees.net — 5150 BIOS Revisions.
  • IBM 5150 with 5151 monochrome display and Model F keyboard
    IBM 5150 with 5151 monochrome display and Model F keyboard
  • IBM 5150 front view, 5.25" twin-floppy configuration
    IBM 5150 front view, 5.25" twin-floppy configuration

See Also


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