After I wrote about the possibility of programmable Mac ROM SIMMs in Quadras a couple of months ago, I suspected that there had been a way for developers at Apple in the 68k Mac era to reflash the ROM in their Macs during development, just like BIOS updates on PCs. The reason I believed this is because the ROM SIMM socket in the Quadras brought out pins for 12V (VPP) and write enable (/WE). I had verified that the write enable pin was going into the memory controller chip in several Mac models, so I was pretty confident that in-system programming was possible.

As luck would have it, multiple people pointed out to me that an Apple internal utility used for ROM flashing had been uploaded to the Macintosh Garden. It was recovered from a prototype PowerBook 520 purchased in 2020. Of course, I had to download this utility and figure out how it works.

I ran it on my LC 475 and this is what it looked like:

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After getting the PWM backlight working in my last post (here are links to parts 1, 2, 3, 4, 5, and 6), there was only one piece remaining for having a fully functional display in my Chumby 8: the touchscreen controller. The display output worked perfectly fine but I couldn’t detect presses on it.

The Chumby 8 and Insignia Infocast 8 have a 4-wire resistive touchscreen:

These aren’t so common anymore — it seems like almost everything is capacitive touch nowadays. I wasn’t familiar with the theory of operation behind resistive touchscreens until I wrote this post. Basically there are two transparent resistive layers. One layer goes left and right and the other goes up and down. When you press the touchscreen they connect together. You can calculate the X and Y positions where the layers meet by driving a voltage across one layer and then measuring the voltage of the other layer. Here’s a nice document that does a great job of explaining it in detail.

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