IBM PC (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 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 standard for personal computers for the following two decades.
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| IBM 5150 Personal Computer with 5151 monochrome display and Model F keyboard | |
| Specifications | |
|---|---|
| Developer | IBM Entry Systems Division, Boca Raton |
| Manufacturer | IBM |
| Type | Desktop personal computer |
| Released | August 12, 1981 |
| Discontinued | April 2, 1987 |
| Intro price | US$1,565 (base, 16 KB, no FDD); typical home/school system around US$3,000 |
| CPU | Intel 8088 @ 4.77 MHz Optional Intel 8087 FPU |
| Memory | Early board: 16-64 KB on-board Later board: 64-256 KB on-board Expandable to 640 KB via ISA RAM cards |
| Storage | Cassette port; one or two 5.25" floppy drives (TM100-1 single-sided 160 KB; TM100-2 double-sided 320/360 KB) |
| Display | MDA + IBM 5151 monochrome display, or CGA + IBM 5153 colour display (separate adapter card) |
| Sound | Internal PC speaker, driven by Intel 8253-5 timer channel 2 |
| Dimensions | 20" W × 16" D × 5.5" H |
| Weight | 21 lb (no FDD); 25 lb (one FDD); 28 lb (two FDDs) |
| OS / Firmware | IBM PC DOS 1.0 through PC DOS 3.3; CP/M-86; UCSD p-System; IBM Cassette BASIC in ROM |
| Successor | IBM PC XT (5160) |
| Codename | Acorn (development project name) |
| Model no. | 5150 |
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. A second socket on the motherboard accepts an Intel 8087 floating-point coprocessor, sold separately.
The CPU is supported by the IBM 8088-family chipset:
- Intel 8259A — programmable interrupt controller.
- Intel 8237A-5 — DMA controller, also responsible for DRAM refresh.
- Intel 8253-5 — programmable interval timer (system tick, refresh trigger, 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 and clearly marked in the silkscreen near the keyboard port.
| Silkscreen marking | On-board RAM | RAM chip type | Banks |
|---|---|---|---|
| 16KB-64KB (early) | 16 KB – 64 KB | 4116 (16 Kbit, requires −5 V) | 4 × 16 KB |
| 64KB-256KB (later) | 64 KB – 256 KB | 4164 (64 Kbit) | 4 × 64 KB |
The motherboard carries ROM in five sockets:
- U33 — BIOS ROM.
- U29, U30, U31, U32 — IBM Cassette BASIC (Microsoft BASIC).
There are four documented BIOS revisions, all dated rather than numbered (date format MM/DD/YY):
| 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/possibly unreleased) | Has a bug where EGA / 5150-compatible VGA fail if SW1 video bits are set ON-ON |
| 10/27/82 | 1501476 | 16KB-64KB or 64KB-256KB | Removes the 1981 BIOS limits; bug: a corrupted HDD expansion ROM at C8000 beeps "1 long + 2 short" instead of displaying "C800 ROM" |
System configuration is set by two banks of DIP switches (SW1 and SW2) on the motherboard. The 5150 has no battery-backed CMOS — the DIP switches are read at every power-up.
Storage and Expansion
- Cassette port (5-pin DIN, rear panel) for tape data, paired with the IBM Cassette BASIC ROM.
- Floppy disk drive — one or two half-height 5.25" drives: Tandon TM100-1 single-sided (160 KB at PC DOS 1.0; 180 KB at PC DOS 2.0+) on the earliest 5150s, then 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+). 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 63.5 W Type 1 PSU. The later 130 W Type 2 PSU made it possible to fit an MFM hard drive on an IBM Fixed Disk Adapter.
- 5 × 8-bit ISA slots.
Display
The 5150 has no on-board video. All video is provided by an ISA adapter card driving a separate monitor:
- IBM Monochrome Display Adapter (MDA) — 80×25 text only, monochrome; paired with the IBM 5151 green-phosphor monitor.
- IBM Color Graphics Adapter (CGA) — text and graphics modes; paired with the IBM 5153 colour monitor or any composite TV.
Keyboard
The 5150 ships with the IBM Model F (83-key) XT-layout keyboard — the original capacitive buckling-spring keyboard. It connects to the rear of the case via a 5-pin DIN connector. Unlike later AT-class machines, the 5150 has no keyboard controller IC on the motherboard — the keyboard interface is 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.
Both produce +5 V, +12 V, −5 V, −12 V on the motherboard's P8/P9 connectors. The −5 V rail is required by the 4116 DRAM on the early 16KB-64KB board, and by the cassette circuitry on the later 64KB-256KB board.
General Maintenance
Cleaning procedures, PSU voltage checks, and preventive care are documented in IBM PC (5150) Maintenance Guide.
Capacitor Replacement Guide
The 5150 uses 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 verified motherboard capacitor information are at IBM PC (5150) Capacitor Replacement Guide.
Troubleshooting
POST beep codes, parity errors, RAM bank 0 failure, keyboard 301 errors, and PSU diagnostics are at IBM PC (5150) Troubleshooting Guide.
Gallery
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IBM 5150 with 5151 monochrome display and Model F keyboard -
IBM 5150 front view, 5.25" twin-floppy configuration
Related Pages
References
- IBM, IBM 5150 Technical Reference (part 6025005, August 1981) — bitsavers/minuszerodegrees mirror.
- "IBM Personal Computer" — Wikipedia.
- 5150 BIOS Revisions, minuszerodegrees.net.
- 5150 Motherboard Revisions, minuszerodegrees.net.
- IBM 5150 Personal Computer, OldComputers.net (dimensions and weight).