RetroChallenge 2021 - ET-3400 ROM replacement

As this my first RetroChallenge entry I thought I'd start with a relatively simple project.

I've chosen to create a small ROM replacement board for an ET-3400 Microprocessor Trainer. As part of this project I am designing and drafting the schematic, testing the circuit design and creating PCB artwork ready for production.

Background:

The Heathkit ET-3400 Microprocessor trainer was sold as a kit or an assembled unit from about 1976. The trainer features a Hexadecimal keypad, 7-segment display, 500KHz 6800 CPU, 256 bytes of RAM, 1K Monitor ROM and a breadboard area for hardware experiments.

more information on the unit can be found here:

http://www.decodesystems.com/heathkit-et3400.html

I purchased a second hand ET-3400 and a later model ET-3400A about 20 years ago and I've joined the ET-3400 group earlier this year - https://groups.io/g/ET-3400

One of the problems with these early machines is that the ROMs are not pin compatible with modern JEDEC pinouts. In the case of the ET-3400 it uses a MCM6830A mask ROM. If you need to patch the ROM or replace a bad ROM there is no direct drop in replacement that is currently manufactured.

Project Aim:

While other people have already created and published replacement boards that can use 2716 EPROMs with additional TTL logic, they are quite a bit larger than the original 24 pin socket.

What I aim to do is create a 24 pin size PCB that uses more modern (and much smaller) surface mount chips so that the whole design will be a compact drop in replacement that has two or more selectable ROM versions on the one board.


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