Hobbyists are rediscovering the appeal of classic microcomputers from the 70s and early 80s, eager to tinker with computers at their fundamental level.

Vince Briel designed the Replica I with permission from the Apple I’s original creator, Steve Wozniak. He sells it assembled ($199), as a kit ($149), or just the motherboard and specialized chips ($79). (Alternatively, the schematic, PCB layout, and fabrication documents — enabling you to build a system from scratch — are included with my book Apple I Replica Creation.)

The Replica I is built around the same 6502 microprocessor used in the Apple I, Apple II, Commodore 64, Atari 2600, and NES. The design has three parts: processor section, video section, and PS/2 interface. (The PS/2 interface is peripheral; an ASCII keyboard works just as well, and can be connected directly to the processor section.)

The processor section consists of the 6502 processor, a 6821 peripheral interface adapter that controls all the I/O, a 32KB RAM chip, an 8KB ROM chip, and three TTL chips. The keyboard provides ASCII input to the 6821, which passes it along to the processor. To display text on the screen, the processor sends ASCII to the 6821, which in turn passes it (as ASCII) to the video section.

The Apple I had no graphics support and was unable to edit text once it was sent to the video section, which served as a dumb terminal. The Replica I uses a Parallax Propeller to send video to TV, and there are hacks to redirect video output to a printer or teletype.

The Apple I is about as simple as an 8-bit microcomputer can get, and hacks and projects for it abound. Larry Nelson has ported a floating point BASIC to the Apple I. Vince Briel designed a serial interface card, ported the classic 70s game Star Trek, and recently added user firmware updates to the Replica I — no need to send in for new chips. Ken Wessen added a full-blown assembler to the EPROM, letting the user assemble 6502 programs right on the Replica I without a PC.

Mike Willegal (willegal.net) sells a replica Apple cassette interface kit for loading programs from tape (he also sells an all-vintage Apple I clone called the Mimeo 1 for, yes, $666.66). Others are replacing cassettes with iPods.

The Apple I Owners Club (applefritter.com/apple1) serves as the meeting place for retrocomputing enthusiasts. Ideas are exchanged, hardware is built, and software is written. If you’re interested, I hope you’ll join us.