ralphb Posted January 14, 2016 Share Posted January 14, 2016 How does inserting a cartridge trigger a console reset, electrically? I know that putting -5V to RESET will reset the console, but I thought that at some point those -5V need to go away again. Looking at a TI cart I basically see this: . Vss Vdd RESET | | | | | | +---??---+---R---+ . What's that mystery component -- looks like a (Zener?) diode? I assume that this will put RESET on Vss after a while, but how does it work? And how do the Super Carts handle this? It seems the red board just permanently connects -5V to RESET?! (BTW, I'm totally confused by negative voltages. Is -5V = Vdd still considered "high", with Vss being "low"?) Quote Link to comment Share on other sites More sharing options...
Stuart Posted January 14, 2016 Share Posted January 14, 2016 There's a capacitor inside the console between the cartridge port reset input and the processor reset circuit. Applying -5V to the cartridge port reset input momentarily pulls /RESET within the console low, then /RESET returns high as the capacitor charges. So connecting the cartridge port reset to -5V permanently is not a problem - you get an initial reset pulse then nothing. When you remove the cartridge, the capacitor discharges through a parallel resistor. -5V does not fit into the "logic low/high" scheme. Applying -5V to a normal logic gate normally results in something breaking ... 1 Quote Link to comment Share on other sites More sharing options...
ralphb Posted January 14, 2016 Author Share Posted January 14, 2016 Thanks for the answer, that makes sense. But what's that diode on the cart board for then? And regarding -5V and high/low: Yeah. But what if we have +5V, 0V, and -5V at the same time? The reason for asking was that Thierry calls RESET "active high", which would only make sense if -5V is considered "high" in this context. (I'm also not used to differentiating between Vss and GND, which again seems relevant for the cart port.) Quote Link to comment Share on other sites More sharing options...
Stuart Posted January 14, 2016 Share Posted January 14, 2016 I suspect the "diode" is actually a smoothing cap on the -5V supply. Is it clear glass, with *no* band at one end to indicate orientation? Is there another identical one for smoothing the +5V supply? Where exactly does Thierry say "active high"? Calling a signal that is essentially -5V "active anything" might perhaps be a little misleading.(I Googled for the pinout of the cartridge port connector and that said "active high" for RESET as well.) Quote Link to comment Share on other sites More sharing options...
ralphb Posted January 14, 2016 Author Share Posted January 14, 2016 I suspect the "diode" is actually a smoothing cap on the -5V supply. Is it clear glass, with *no* band at one end to indicate orientation? Is there another identical one for smoothing the +5V supply? Bingo! Never seen a cap like this ... Quote Link to comment Share on other sites More sharing options...
+Ksarul Posted January 14, 2016 Share Posted January 14, 2016 Glass caps were quite common in the period around 1980. . . Quote Link to comment Share on other sites More sharing options...
ralphb Posted January 15, 2016 Author Share Posted January 15, 2016 I remember playing with the innards of a broken TV set when I was a small kid. Of course the only meaningful thing we could do with something like a resistor or a capacitor was to cut it into two parts ... This probably contaminated me for life! Well, this year I'm set to correct my hardware deficits ... 1 Quote Link to comment Share on other sites More sharing options...
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