KB10004: IBM PC, XT, and AT Overview

IBM PC and XT

The original IBM PC (5150) appeared in 1981 with an 8088 processor, optional floppies, and an open architecture that spawned the clone industry. The PC XT (5160) followed in 1983 with a built-in 10 MB hard drive and eight expansion slots. Both used the 8-bit ISA bus. The XT-286 (5162) arrived later as a cost-reduced model with an 80286 in an XT-style case.

IBM PC AT

The AT (5170) launched in 1984 with the 80286, a 16-bit ISA bus, and support for up to 16 MB RAM. It introduced the keyboard connector style that endured for decades, a battery-backed CMOS for configuration, and higher-capacity floppies. Clock speeds increased from 6 MHz to 8 MHz in later revisions.

Compatibles

Third-party clones replicated the IBM design at lower cost, often with small improvements. Taiwanese manufacturers produced many of these systems. Compatibility varied; some clones had quirks with certain software or expansion cards. The ISA bus and DOS compatibility made much of the ecosystem interchangeable.

Common Restoration Notes

Power supplies in these machines use electrolytic capacitors that fail with age; recapping is standard. The CMOS battery on the AT can leak and damage the board—remove or replace it. Hard drives from this era often have stuck heads or failed bearings; handle with care. Floppy drives need cleaning; belts may need replacement. Expansion cards can suffer from contact corrosion; cleaning the edge connectors sometimes restores function. Verify PSU output before powering on.