A simple, failure immune 4-bit digital computer made out of common TTL chips

by SV3ORA

Introduction

I always wanted to build a simple computer system out of common TTL chips but the complexity and cost was the main obstacle. Eventually, I found a project on the homemade CPU ring which described the construction of a 4-bit CPU out of TTL chips. Despite most of such articles are quite complex to understand and confusing to navigate through their pages, this article was so well written and so complete that I could entirely understood how the CPU worked on a single website reading. This inspired me to build my own version of it.

This CPU used mostly common TTL chips apart from the EPROMs and the RAM chips. I decided to build my own militarized version of it first and later on to try to make modifications to it. The main modifications which I felt I needed to make, were to replace the EPROMs and RAM, to use entirely TTL or more standard parts. This is a project that is probably going to take years to complete properly.

This is a slow and very limited usage computer system and it has been basically rebuilt by me for educational purposes and components reliability demonstration. There are not such things as keyboard and screen, if you want a such computer, build the ZX-80 that I have in another section on my website, or other such computers that I will include on the website. You may think of it more like a microcontroller rather than a computer. It is programmed to do a single job, using machine language code entered mainly using front panel switches, much like the MITS Altair 8800. There is no prepackaged CPU though. The CPU is mage out of common TTL chips.

There is no need to repeat everything here. Download an read the project articles first, to understand how the computer works. Then follow my page below to see how to make a rugged version of this computer, that uses more standard parts, it is failure immune and can work for many years of reliable operation.

Electronics Parts List

There was no electronic components parts list for this project. There were some gates that were not marked in the schematics the author provided, so I decided to make my own parts list by carefully examining these schematics. Since I was interested in immunity to failures due to physical factors, I used more rugged components. Here is the parts list including the "missing" TTL chips. The actual components used are listed. You may substitute some of them with your versions if rigidity is not your goal. For example you can substitute the F series semiconductors with the LS series and the military switches with cheap ordinary ones.

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