In the world of retro computing, where nostalgic gamers reignite their passion, the Amiga stands as a titan of vintage technology. My latest project has taken me on an epic journey to create a ROM switcher for these beloved machines. This isn’t just an upgrade; it’s a labor of love to preserve and enhance a classic.
The Challenge: Building a ROM Switcher
I set out with a simple goal: to build a straightforward, passive ROM switcher for the Amiga. The challenge arose from the fact that not all Amigas were made equal. The early revisions of these machines had a hardware bug where the ROM socket wasn’t correctly wired. Specifically, address line 17 was connected to Byte select, which wasn’t an issue with the small ROM sizes of Kickstart 1.3 but became a problem with later, larger ROMs.
My mission was clear: create a ROM switcher that is compatible with all 68k Amigas with OCS or ECS chipsets—A500s, 600s, 2000s, and CDTVs. I wanted to switch between at least two ROM images, and that’s where my prototype journey began.
The Prototype and Its Evolution
I started with a 27C800 chip, allowing for two ROM images. My initial prototype worked functionally but was a bit of a hack job. I even considered adding LEDs to indicate the selected ROM, but that idea didn’t pan out. The key missing feature was compatibility with early revision Amigas, where the ROM socket was miswired. By correcting this in the switcher, I aimed to create a truly universal solution.
After proving the concept with my crude prototype, I moved on to a more polished design. The first iteration was functional but had some shortcomings, like the floating bite mode select line in early revision mode. This line needs to be high to enable the 16-bit word mode that Amiga runs by default. By adding a bodge wire to tie that pin directly to VCC, the switcher worked perfectly.
Refining the Design
The second revision of the board addressed these issues and extended the functionality. It supported a 27C160 chip, allowing for up to four ROM images on a single switcher. These ROMs are relatively easy to come by, provided they are no slower than 100 ns. This version came very close to what I envisioned, but I still had a few tweaks to make.
For instance, the new revision couldn’t program ROM images through the switcher—a feature that existed in the first revision. This was because the byte select pin, now tied directly to VCC, couldn’t handle the programming voltage. However, I saw potential in this functionality. It would allow for easier ROM image management, eliminating the need to manually combine images or navigate the complexities of ROM writing software.
The Final Version and Beyond
With the final version of the ROM switcher, I achieved my goals: it fixes the early revision Amiga issues, allows for easy writing of ROM images, and supports multiple ROMs. The switcher can hold four ROMs and even allows for writing new images individually, making it flexible and user-friendly.
In the end, the project was more than just about creating a piece of hardware. It was about preserving the legacy of the Amiga and making it more accessible for enthusiasts and collectors. Along the way, I also explored building an adapter for the 27C4096 chip, a more straightforward and logical design than the 27C400. This new adapter worked perfectly on the first try, a rare occurrence for me!
A Word of Thanks
I’d like to give a special thanks to PCBWay for sponsoring this project. They offer PCB prototype fabrication, CNC machining, and 3D printing services, all of which were instrumental in bringing this project to life. Their support made it possible to turn my designs into reality.
If you’re interested in building your own ROM switcher or need other PCB services, check out PCBWay.com. And if you’re curious about the ROM switcher, it will be available on PCBWay’s shared projects.
I would also like to than Chris “Cathers” for providing the ROMs and the inspiration for the 27C4096 adaptor and donating an Amiga 1200 case to the channel 🙂
Stay tuned for more adventures in retro computing, and if you’re curious about my process or other projects, check out my other videos!
This was my journey creating a universal Amiga ROM switcher. It’s been a fascinating and challenging project, but seeing it work perfectly in the end made it all worthwhile. If you’re a fan of vintage technology or just love a good DIY project, I hope this inspires you to dive into the world of retro computing.