RetroChallenge 2021/10 - ET-3400 ROM replacement

I’ve decided to enter the RetroChallenge 2021/10 and I chose to create a simple ROM replacement PCB design for my 1977 Heathkit ET-3400 Microprocessor trainer.

my RetroChallenge blog is at

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I’ve made quite a few updates on my retroChallenge entry. This is just a summary of what I’ve done so far:

  1. stated the aim of my project.
  2. defined the problems with replacing the stock ROM.
  3. proposed a couple of programmable ROM options and selecting one.
  4. drafting the schematic.
  5. building a prototype on breadboard.
  6. editing the ROM firmware.
  7. programming an EEPROM.
  8. testing the circuit on the ET-3400.

details and lots of pictures on my blog.

next phase will be further tests and designing the PCB artwork.
I have added an extra challenge for myself after that’s all done just for a bit of 6800 7-segment LED display programming fun.

Mark C.

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Since my last update I’ve done the following:

  1. tested /CE and /OE chip select options.
  2. planned the PCB layout.
  3. worked out which SO package to use.
  4. added extra features, aka Feature Creep.
  5. fixed up the circuit diagram and more tests.
  6. managed to squeeze in the extra features onto the PCB.
  7. drafted the PCB layout incorporating test results.
  8. produced PCB-3D view and uploaded gerbers to JLCPCB for better PCB images.
  9. got side-tracked for a couple of days while I designed a programming adapter for this project. more feature Creep !!!
  10. created the gerber files, schematic file and description and uploaded to the files area.

40 PCBs will be ordered soon, but not in time for the end of the RetroChallenge.

last thing I’m going to try in the remaining time is writing a small demo program in 6800 code to finish it off.

Mark C.

Great work Mark! Keep it up =)

I’ve added a few more posts and my final blog to my RetroChallenge entry.

  1. obtained the tools for the programming task.
  2. scaled back programming task and successful assembly of program.
  3. recorded a video showing program running on ROM replacement breadboard prototype.

Although my RetroChallenge blog is done, there is still more to do on my project. I have to order the PCBs and finish designing and building the programming adaptor.


Fantastic work! Look forward to see what happens in the future.

thanks for the awesome information.

A short follow up on my project…

I’ve had 40 PCBs made and delivered in early January. I put the first one together with a pre-programmed AT28C64 SOIC-28 EEPROM, since I haven’t made the programming adapter yet.

It’s been running fine for the last 7 weeks and here is a few pictures of the end result.

close up showing low profile lead frame pins vs standard ROM:

74LS20 fitted, installed in ET-3400 and running:

I haven’t ordered the programming adapter PCB yet as I’ve been working on a couple of other projects. So the only way to program this at present is to do it before soldering on the EEPROM.

Mark C.