A new project I have started working on involves the Apple IIgs. It was Apple’s last 16-bit (and 8-bit) computer. Inside are many application specific integrated circuits, or ASICs, that make the IIgs an extraordinary member of the Apple II family. One chip, in particular, is called the “MEGA-II.” This chip takes all of the individual logic chips from the original Apple II design and incorporates them into a single 84-pin PLCC.
The project I have in mind needs the MEGA-II. I need to design some printed circuit boards for it and a few other IIgs chips. That goal means I need at least one custom Kicad schematic symbol. I plan to create a custom library of Apple IIgs components.
Like other computers from the same era, complete schematics are available. However, they are not in a modern format. Since I need to create symbols for so many of the chips as it is, I may end up re-creating the entire IIgs schematic.
For now, here is the process I use to create custom KiCad schematic symbols and parts.
Tutorial on schematics basics
The funny thing about schematics is that they are much easier to draw than they are to read. There are many common circuits. When an experienced engineer looks at them, it’s like a second language. When someone less experienced looks at them, it looks like random lines and symbols thrown together at the last-minute. (Or maybe that’s just the schematics *I* draw.)
Other than reading Schematic Symbols themselves, one of the basic skill necessary to read a schematic is recognizing series and parallel circuits.
In short if the same current flows through all the parts, they are in series. While if current has different paths, they are in parallel. Keep reading to dive into this tutorial on how ohm’s law applies to series and parallel circuits.
Engineers make a schematic to explain their circuits.
One time I was looking for a non-tourist pub in Japan. I asked someone for help. She said, “I’m sorry, but I do not speak good English. I will bring my friend, and she will draw you a map.” (Exact quote!) The map her friend drew, gave directions to a bar with a “Neon Yellow Sing.” She meant “sign.”
The map was the method we used to communicate with each other, even though we didn’t both speak English. With this crude but useful map, I could find my next
drinking place destination.
Schematics are the same as this map. Even if you don’t speak the same language, you can communicate how a circuit works when you make a schematic.
Use one of these five tools, when you need to a document a circuit or when you need to ask for help.
When you start creating your own parts in EAGLE, you’re going to want to store them somewhere. Here’s how to setup EAGLE to use custom directories and how to create your own Library, which gets stored in the custom directory. There are a number of steps involved, but once setup custom libraries give you a place to store components you create as well as the ability to copy other parts into your library.
Over the past several months I have been kicking around a new way to teach non-engineers about electronics. My idea is to use simple language to explain complex engineering subjects. Each video will be in the range of 3-5 minutes. While putting together the first couple of topics I started to realize I wanted this to be something to stand on its own.