Drawing Apple MEGA-II Breakout in KiCad

Mega-II Partial Symbol

There is a project that has been sitting on my “to do” list for too long. My lab notebook has several dedicated pages for it. But I have not made progress. I decided to take some advice I have given to other people. When you’re stuck on starting a task, break down the project until you find a piece small enough you can get it done with no problem.

The project involves the Apple IIgs. It was Apple’s last 16-bit (and 8-bit) computer. Inside are some application specific integrated circuits or ASICs that make the IIgs. The name with my attention is named “MEGA-II.” It takes all of the individual logic chips from the original Apple II design and incorporates them into a single 84-pin PLCC.

MegaII from AppleLogic 960px

Image from Applelogic.org (Full)

I need to design some printed circuit boards combining the MEGA-II and a couple of other chips. My current step is creating a library of custom components in KiCad.

With the help of this Apple II ASIC listing and a few other resources, my project will capture the entire IIgs schematic. Of course, by the time I finish, someone will have made a python-vision script to convert these scanned schematics to KiCad automatically.

In the spirit of sharing, I streamed part of my schematic capture process. I started with a break out board for my custom MEGA-II symbol. During the stream, I realized the chip I was working with was a PLCC style. There are sockets for this package type. That realization led me to find that I could buy PLCC break out boards. Why re-invent the wheel? Also, using sockets means I can prototype with fewer MEGA-II chips. Using fewer chips means cannibalizing fewer computers.

Getting as far as I did was not a waste. Since I plan to re-create the entire schematic anyway, I can use what I made already. Also, eventually I will add other ASICs to that initial board.

What is the Apple IIgs MEGA-II Chip?

In this clip, I explain that the Apple IIgs included backward compatibility with the earlier 8-bit Apple-II line of computers. Apple’s engineers combined all of the individual logic chips that make up an Apple-II and put them into a custom ASIC. If I add a 6502 processor, some RAM, and an Apple II ROM, I have the basics of an Apple II computer.

Watch Highlight: Introducing the Apple IIgs’s Mega-II chip from baldEngineer on www.twitch.tv

Related: Realizing the socket

At this point in the stream, I realized I could socket the chip. Up until this point, I assumed I would need be soldering the ASIC to a breakout board. As I said earlier, sockets are going to save me chips (and time.)

Watch Highlight: Realizing the MEGA-II should be socketed. from baldEngineer on www.twitch.tv

What is the project?

You might be wondering what project I have in mind. I will leave that open for the moment. I do have an end goal with these special chips in mind. But I’m not ready to say what just yet.

Long comments, URLs, and code tend to get flagged for spam moderation. No need to resubmit.

Leave a comment