> Normally, I would expect floating address pins to be completely random, but this was very consistent.

I think “random” assumes some kind of perfect conditions. There will be leakage current to ground, leakage current from Vcc, and the actual threshold between high and low for the input will be some particular value.

Just a quick look at the data sheets for the SN54LS670 from TI, and I see that the output pin leakage current is rated at no more than 20 µA. That’s worst-case, but if you apply Ohm’s law and plug in 3.3 V, you get something on the order of 160 kΩ output impedance. I have a limited understanding of electronics and I’m sure I’m missing something, but I’d expect a lot of the output pins to settle on a reasonably stable voltage when they are turned off. Maybe natural variations in geometry or chemistry will push some pins higher and other pins lower.

You can run an entire CPU core with less than 20 µA current these days, just to give it a sense of scale.

> [About buying a used cartridge] In retrospect, after all this adventure, that doesn’t seem like much, but “spending fifty bucks instead of learning something” is not the name of this blog.

Absolutely love this quote.

Very interesting post. I played some MSX recently and there’s some good games there. I’m sure I’m going to end up with an MSX2 some day.

I recently started making some SEGA master system cartridges. Your only option there is to destroy a Monopoly or After Burner. :-|

The Maze of Galious is such an underrated game in terms of IP.

I had the pleasure of attending MSXGOTO40 last december.

Great explanation what megarom actually means, I never knew