I want to know what the 555 timers are for...
Joystick port. One 555 for each axis. Charge up a capacitor, discharge it through the potentiometer in the joystick, and use the CPU to count how long it takes to discharge. There was a ROM routine (and a BASIC command) that did all of that for you, but you could write your own code if you wanted to measure resistance values beyond what would've been used in joysticks.
Other Apple IIs used a 558 to support two joysticks. I started with a IIe in 1985, which I upgraded to a IIGS (by way of a motherboard swap) around 1992 or '93. Still have it, as well as some more Apple IIs I've picked up cheap through the years. Never had a IIc, though.
Those old tricks seem interesting. I wonder if they provide a DAC able to put analog inputs on them.
I never had a pre PC Compatible computer. My first computer was a Pentium 100MHz. But I love retrocomputing and retrogaming in an obsessive way, it's part of my personal madness to 80s and early 90s stuff, but I like these classic chips too.
My personal dream is design highly functional, robust, innovative and cheap OSHW for vintage systems. 99% designs are propietary, some of the most interesting ones use somewhat complex chips (SRAM, FLASH, FPGA, CPLD) and the most complex ones are those that reimplement original Hardware as replacement chips improvements (this happens specially in video chips, see, VBXE2 for Atari XL/XR and for Amiga OCS/ECS/AGA) to most of the time they also add features and provide more modern video photos proud outputs such VGA/DVI/HDMI with different scaling modes)
Apple IIGS! The inspiration of Super Nintendo/Famicom! It has a very good audio chip!
http://www.callapple.org/software/announcing-apple-iigs-system-6-0-2/https://youtu.be/Wu9a724Az3ENaughty Dog started developing on it.
Dave, please make a good teardown of it if you can! It's an American machine and not so easy to get, but maybe you're lucky!
I only found the following, not do good teardowns like yours
Like in other systems, there are expanding. Both old and new:
Some games:
https://youtu.be/IsXPn6OCMF8There's some demoscene:
And nice music:
Some videos and ads:
I liked your X68000 teardown a lot. It's an amazing machine with tons of excellent arcade ports, some exclusive games and lots of those horny cartoon games (hentai, eroge). It has a bunch
Here we have the modest Sinclair ZX Spectrum 128K, maybe I'll send one if get money next year plus some (hopefully) own Hardware design.