> 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.
> Your only option there is to destroy a Monopoly or After Burner.
Not sure what you mean. krikzz sells a Master Everdrive if you want something suitable for multiple games. If you want a single game cartridge for a homebrew release, raphnet has you covered.
The one exception is if you have a Japanese Master System (or Mark III). These have a different cartridge connector that's shared with the SG-1000 and SC-3000 and flash carts are harder to find. You may be able to use a western SMS flash cart with a passive adapter though
I’ve already got an Everdrive. I like making single game carts. I was unaware of the raphnet product but that’s considerably more expensive than original carts.
It doesn't handle all the features of the standard mapper so not compatible with all games, but it's designed to be cheap to manufacture. Unfortunately I don't think anyone is selling these so you'll need to take the files to a PCBA house (or get bare PCBs manufactured and do assembly yourself)
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.