ET-3400 ROM - Building the Prototype

To build the prototype I can't easily use SMD components on a breadboard without appropriate adapters and I don't have any suitable ones. I have several 28C64B EEPROMs in SOIC-28 format that I purchased for another project and luckily I also have one in DIP-28 size. Since these are essentially the same part in different sizes I can use the DIP-28 EEPROM for testing the circuit.

Also I don't have any 74LS20's in SMD size yet, but I can use a 74LS20 in DIP-14 size for the prototype.

That solves the parts requirements, but unfortunately my ET-3400 has another in-progress breadboard project on it already and I don't want to pull it all apart yet as I haven't finished it.

This shows how full my ET-3400 breadboard is already. You couldn't squeeze an ant in there.

I decided to use an old Wish breadboard I have. The yellowing plastic on it qualifies it as a retro part by itself. It might be old, but it's a lot better than some of the garbage breadboards they make these days.

While I was wiring up the prototype I discovered a couple of minor errors on my circuit diagram that were caused by cut-n-paste and not checking. We've all done it!

While checking your own work is not a good practice, as retro computing hobbyists it's not always easy to find someone to check it for you. One of my methods of self error checking a circuit diagram or a PCB overlay is to mark off each track in pencil as I fit it or cross check it. Notice the pencil lines on all the nodes, and I've circled the errors that I found.


the ribbon cable has a 24 pin DIL crimp headers on each end so that it will connect this breadboard to the ET-3400 ROM socket. That's sure to clog up any remaining space on my ET-3400.

next I have to program the EEPROM before I can plug it in...





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