Introducing Bit Preserve

Recreating vintage schematics with KiCad

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A project I work on in my spare time is creating a portable Apple II. Like many of my projects, one leads into another. I started out wanting to make a mobile Apple II, and now I’m working on a project called Bit Preserve. How did I get from one project to the next? Well, as I looked into how to make a portable Apple II, I realized a significant issue. The original Apple II logic board has almost 80 ICs. Being a design from 1975, they are all through-hole packages. The good news is that except for the ROM chips, they are all off-the-shelf components. But such a size means it might be impossible to turn it into something handheld. I almost abandoned the project. Then, I learned about a chip included in the Apple IIgs. The name of the ASIC is “MEGA II.” (Nothing to do with Arduino.) It is a chip that integrates all of those off-the-shelf chips into an 84 pin package.

As I dove deeper into the project, I realized I needed other support chips to make the MEGA II useful. There is a decent book that discusses the technical details of the Apple IIgs, but it does not get into chip or board level design. For that detail, I had to look at the original schematics. While I am ecstatic that someone archived these original documents as PDFs, I quickly became frustrated. Sometimes the scan quality is not very good, and it is nearly impossible to search for symbols across multiple pages. I thought to myself, “There has got to be a better way!”

Bit Preserve on GitHub

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What is the Apple IIgs? – AddOhms Live Clip

Bald Engineer Twitch Channel
2018-11-18

Watch What is the Apple IIgs? Highlight | AddOhms Live from 

During a live stream, I was asked: “What is the Apple IIgs?” In this AddOhms Live Twitch Clip, I answer the question.

The Apple IIgs was the last of the highly successful Apple II line of computers. The “GS” stood for “graphics” and “sound.” Compared to previous Apple II computers, the IIgs was a fully 16-bit machine. When connected to its proprietary RGB monitor, it rendered a gorgeous display. Sadly, not much software took advantage of the improved graphics and sound capabilities. The IIgs was fully backward compatible with the older 8-bit line of Apple II computers. Its compatibility was so good that most IIgs users only used it in the compatibility mode.

How did the Apple IIgs achieve backward compatibility?

The IIgs contains an ASIC called the “MEGA-II.” (Which has nothing to do with the “Mega” Arduino boards.) It includes all of the individual logic chips from the original Apple II design as a single IC. Well, in addition to that IC you also need to add a CPU, RAM, and a ROM.

In my opinion, the Apple IIgs is best of the Apple IIs. In fact, of computers in that era, it is my overall favorite. When I got the IIgs, it replaced my previous pick: a Macintosh SE/30.