the design is not an ADC. it uses a quad 555 timer with RC circuits composed of the external joystick resistance and 22nF capacitors. the Apple IIe just measures the time it takes to exceed the threshold.
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the design is not an ADC. it uses a quad 555 timer with RC circuits composed of the external joystick resistance and 22nF capacitors. the Apple IIe just measures the time it takes to exceed the threshold. 12 comments
i removed the 'ls125. there was definitely a puddle of electrolyte underneath (it crackled and fizzed during desoldering). note that the potentiometer connection (red arrow) is right next to a pin tied to ground (blue arrow). electrolyte is conductive, so maybe it was creating a leakage current.
[DATA EXPUNGED]
@tubetime Did you figure out what the ‘LS125 is doing there? I looked it up and guess it’s just a buffer. https://www.ti.com/lit/ds/symlink/sn54ls125a.pdf @tubetime Ahhh, so IBM _wasn't_ the only company that used an RC circuit as an integrating ADC :'D! |
it's just barely pegged at 250 or so. 2.8ms. the resistance is 87.8K, so this RC curve implies a capacitance of 68nF. that can't be correct.