8-bit SAP-1 Breadboard Computer

I recently came across Ben Eater’s YouTube Channel, the video in my recommendations was about his new project, the 6502 based breadboard computer. It’s a whole series of videos where Ben walks us through creating a 6502 Microprocessor-based computer on a breadboard, connecting a LCD to it, and interfacing using PS/2 and USB. I definitely recommend you watch that series if you want a good understanding of how computers actually work. The 6502 processor from MOS Technology found its way into popular video game consoles and personal computers like Atari 8-bit family, Apple II, NES, Commodore 64, etc.

This post is not about the above computer, it’s about Ben’s 8-bit beadboard CPU project. This is a series where he walks us through creating our own 8-bit CPU using basic 74LS series ICs and EEPROMs. I ordered all required electronic components online and watched his entire video series. When I received all the parts, I started building it one module at a time. Below are some pictures from the build.

While completing this build I encountered some problems that Ben never mentions in his videos, other people who attempted this build would also tell you that Ben’s design has some flaws that need to be addressed. I made the following changes which were not covered in Ben’s videos. 

  • Added 1k resistors to all LEDs.
  • Replaced 10k resistors on the bus with 1k ohms.
  • Tied all unused inputs low/high.
  • Added 220uF capacitors to all power rails.
  • Isolated clock from RAM write RC circuit using two inverters (NAND).
High Level Overview
High Level Overview, Image credit: Ben Eater
Control Logic
Control Logic, Image credit: Ben Eater

If I had to do this again, I would certainly go for better breadboards this time. I used cheap ones from Aliexpress, they worked fine as such but are sorta unreliable, the power rails also don’t run along the entire length and jumpers need to be added. I found these ones on Amazon and they were discounted at $12 for 3. The quality of these boards is actually pretty decent and they aren’t too expensive either. I will use them for my next build, looking at you 6502 😉

Reddit post about the completed build. If you are going to attempt to build this yourself, read this post to save yourself from a lot of headache.