{"id":2832,"date":"2024-06-15T04:21:53","date_gmt":"2024-06-15T04:21:53","guid":{"rendered":"https:\/\/www.kernelcrash.com\/blog\/?p=2832"},"modified":"2024-08-30T21:19:30","modified_gmt":"2024-08-30T21:19:30","slug":"appleii-vga-and-emulating-other-retro-video-subsystems","status":"publish","type":"post","link":"https:\/\/www.kernelcrash.com\/blog\/appleii-vga-and-emulating-other-retro-video-subsystems\/2024\/06\/15\/","title":{"rendered":"AppleII-VGA and emulating other retro video subsystems"},"content":{"rendered":"

Recently I came across the AppleII-VGA<\/a> project by Mark Aikens. It takes a Raspberry Pi Pico, adds some level converter transceivers, then attaches it to the address, data and control signals of an Apple II, and effectively it sits there doing two things. 1. It listens for ‘writes’ coming from the 6502 CPU to the memory where the Apple II thinks its video memory is and 2. It converts those writes into a pixel perfect VGA signal. So it’s like a video output converter that bypasses all the video logic in an Apple II that normally puts out a composite video signal, and instead it converts directly from writes to video memory and creates a perfect VGA image. There’s a fork of it too called \u22002 Analog VGA<\/a> that adds some additional features.<\/p>\n

<\/p>\n

Of course for someone who emulates ROMs and floppy chips, this is quite a cool project so I had to try it. There is a proper PCB for this, but I just hacked it together on a breadboard and added some jumper wires and used one of those MSX prototype boards<\/a> I ordered heaps of some time back (though you have to cut some obvious traces to use on anything other than MSX). So the Apple II has 50 pin edge connector slots and conveniently MSX has a 50 pin cartridge slot.<\/p>\n

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Anyway, when I first hooked it up, it didn’t work at all. Doh!<\/p>\n

My spider sense thought “maybe the four 74LVC245’s I had were not 100% genuine” . Fortunately I had four I had bought at my local Jaycar for another project. So I swapped them in and voila:<\/p>\n

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Loderunner never looked so good.<\/p>\n

Actually it looks like I am running an emulator.<\/p>\n

Is this cheating?<\/p>\n

Probably \ud83d\ude09<\/p>\n

Anyway, note to self “Unbelievably cheap 74LVC245s from aliexpress = Yeah right<\/em>” and “74LVC245s from my local Jaycar = most likely genuine<\/em>“. Why on earth anyone would go to the trouble of making 74LVC245’s that don’t quite work properly is beyond me. I did actually take a zoomed in look at the top of these fake-ish chips and they have the tell tale rough surface … that I had never noticed before. I guess my ‘guess’ relating to why it wasn’t working was because I also used some 74LVC245s to make a pitubedirect<\/a> and they didn’t quite seem 100% either. And those 74LVC245s were part of the 10 or so I ordered off aliexpress. I’ve since ordered quite a lot of 74LVC245s from mouser\/rs-online\/element14\/whoever , and at least they should be genuine.<\/p>\n

Anyway, the AppleII VGA worked in an Apple II<\/em>, but that got me thinking “How hard would it be to adapt it to other older computers?”. So then I sat down and stared at the source code for some time and discovered the world of PIOs. They are clever but weird. The rough way the Apple II VGA works is:<\/p>\n