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.

Schematics and PCB

There were schematics on the author website but these were only images. Also there was no PCB for the project. I contacted the author if he had any CAD files, but I did not get a response, so I have decided to make my own schematics and PCB.

Also the schematic provided by the author was not 100% complete. There are some undefined points, like the power lines, decoupling and enable signals on some chips. These schematics were perfect to understand how the computer works but they were no good for production. This motivated me to make a complete schematic of the computer.

Current version of unfinished schematic (27-4-2012, Eagle CAD 5.7.0)

My version will include monitoring of the I/O with LEDs as well as some other possible changes. Note, I have not finished the schematic yet, so please be patient until I do so.

There was a great decision I had to get in building this computer. Build the whole thing onto one PCB or split all computer modules into different PCBs. Splitting modules according to their role on the computer system gives greater flexibility. One can change modules by replacing them with more updated versions, as long as I/O connections stay the same, without having to replace the whole PCB of the computer. On the other side, lots of cabling is required between different modules. We are making a PCB for this computer because we want easier construction with less cabling, which will lead to a more rugged system. Why to build PCBs if you have lots of cables to interconnect them?

I have decided to go somewhere between the two solutions, by splitting only some of the modules on separate PCBs. Since I wanted to replace the RAM and EPROM with discrete components later on and since these would be quite complex circuits, I thought it would be better to include them in separate PCBs. The rest of the computer logic is small enough to fit on one PCB.

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