Whether you want to build your own C74-6502, or just learn more about its inner workings, this is the place to dig deeper. This section provides detailed explanations of the circuitry in each PCB of the project, as well as a collection of useful downloads and all project files. You may also want to have a look at the Original Build Thread, where design details were discussed live as the project developed.

If you find this content useful, please send us a note to let us know it’s working. Any suggestions to improve the clarity or completeness of the information are appreciated.


  • Block Diagram
  • Microcode
  • Card A – Registers, 6510 port, clock and pinout
  • Card B – ALU, CU, Undocumented Opcodes (UFO)
  • Card C – K24 Address Bus Extension (ADX), SPI interface
  • Card D – C74-SBC Single-Board-Computer Test Environment
  • Card S – C74 Host Socket Adaptor
  • Optimizations

Useful Documents

Project Files

  • Schematics (Eagle CAD .sch)
  • PCB layouts (Eagle CAD .brd)
  • PCB Assembly Guide (showing part numbers .pdf)
  • PCB Boardview (showing silkscreen labels .pdf)
  • PCB Gerber files
  • PCB BOM files (.xlsx)
  • Microcode ROM assembler source files (.65S)
  • Microcode ROM binary files (.65b)
  • Microcode ROM Logisim files (.txt)
  • Logisim Model (6502 V11.circ)

[Download C74-6502 Project Files v1.0]

NOTE: The prototype boards (V1.0) were patched extensively during debugging. All testing for the project was conducted on these patched prototype boards. The patches are detailed in the C74-6502 PCB Patch Guide (v1.0), and in annotated (V1.0) schematics — both of which are included in the Project Files Download above. In addition, updated Eagle files (.sch and .brd) which incorporate these patches are also included in the download (found in the “Eagle Files — Patched” folder). The Patched files have been carefully verified but have not been fully tested. Many thanks to’s ttlworks for producing these updated Eagle files.