8-bit SAP-1 Breadboard Computer

July 30, 2021

One day I came across Ben Eater’s YouTube Channel, the video in my recommendations was about his project, the 6502 microprocessor-based breadboard computer. I found he has a whole video series  about the same where he walks us through building the computer, connecting a LCD to it, and interfacing using PS/2 and USB. 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. I was amazed, this was my introduction to his channel.

This post is actually not about the above computer, it is 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 other components. I ordered all the required items and watched his entire video series. Upon receiving the parts, I started building them one module at a time.

While completing this build I encountered some problems that Ben apparently never experienced in his videos. Others 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 the original videos. 

  • Added 1k resistors to all LEDs.
  • Replaced 10k resistors on the bus with 1k ohms.
  • Tied all unused inputs low or high (to get the output always 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.

Reddit post about the completed build. If you are going to attempt this yourself, read this post

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