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PostPosted: Fri Oct 06, 2017 7:38 pm 
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This was hastily made, and I might be doing it wrong, but would this schematic produce a joystick that could be switched from Atari 7800 mode to Sega Mastersystem mode with a DPDT switch? If not what would I change to make it work?


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PostPosted: Fri Oct 06, 2017 8:34 pm 
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So, quickly summarizing:

DE9 female for reference
Code:
5 4 3 2 1
 9 8 7 6

7800 joystick:
Code:
R → ← ↓ ↑
 L G 5 C
Pins 5 "R" and 9 "L" connect to pin 6 "C" when pressed. Internal resistors pull pins 5 and 9 to pin 8 always.

SMS control pad:
Code:
5 → ← ↓ ↑
 2 G H 1
Pins 6 "1" and 9 "2" connect to pin 8 "G" when pressed.

Ok, with that out of the way...

You can't leave the 520Ω resistors in the way when in SMS mode; the pull-ups inside the SMS aren't strong enough to overpower them.

I'm not certain this can be done with only a 2P2T switch. It seems like there's four different things that need to be moved, depending:
[0P0T] Button "2"/"L" can always have one side connected to pin 9
[1P2T] Buttons have a common node that need to be connected to pin 6 or pin 8
[1P2T] Button "1"/"R" needs to be connected to pin 5 or pin 6
[2P1T] Strong-ish pulldowns need to be enabled/disabled


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PostPosted: Fri Oct 06, 2017 10:13 pm 
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That's what I was wondering about. If I could leave the resistors in place since they aren't a problem in 2600 mode. I guess not. I'll think about it and see if I can use a different switch since pin 8 needs to lose the resistors in SMS mode.

Sorry, Easyeda is limited and I couldn't find the appropriate switch on there so I had to add a second switch to represent one appropriate switch. Is this better? Also I have mixed the buttons up here, but you get the general idea.


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PostPosted: Fri Oct 06, 2017 11:06 pm 
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Um... let's see, that's still off...

In that, R1 can't ever be disconnected. And pin 9 isn't the one that needs to be switched.

It's probably better if you throw away your current schematic and redraw from the specifications instead of trying to adapt your current one—it's harder to identify what's currently wrong and figure out how to change it than to just start from something empty.


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PostPosted: Fri Oct 06, 2017 11:59 pm 
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Oh ok. I didn't know if Pin 5 being tied to ground via R1 was a problem in SMS mode because the pin is unused.


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PostPosted: Sat Oct 07, 2017 12:12 am 
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Super-Hampster wrote:
I didn't know if Pin 5 being tied to ground via R1 was a problem in SMS mode because the pin is unused.
Derp. Confused me.

Schematics I can find of the SMS actually show that that pin usually does provide 5V ... and it's definitely always 5V in the Genesis. So putting a permanent 620Ω there means you'll be drawing 8mA extra power constantly... which is suboptimal but very likely safe.


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PostPosted: Sat Oct 07, 2017 2:06 pm 
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Yeah, I figure with the resistor there it's not shorting out, so hopefully that will be safe.


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PostPosted: Sat Oct 07, 2017 2:08 pm 
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I've worked through two different ways you could do this with a 3P2T switch, but I haven't seen a way to use only a 2P2T switch yet.


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PostPosted: Sat Oct 07, 2017 2:16 pm 
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The switch is unimportant. What ever works. I would also like to point out that I would like to keep SMS mode compatible with other machines that use identical 9 pin connectors.


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PostPosted: Sat Oct 07, 2017 2:55 pm 
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Do we have a comprehensive list of all the consoles that use the DE9 mostly-digital gameport?

I know the 2600 uses this (and I think the C64 uses the same?)
Code:
X → ← ↓ ↑
 Y G 5 1
where X and Y are potentiometers to "5" and capacitors to either "5" or "G" ("AC ground")


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PostPosted: Sat Oct 07, 2017 4:24 pm 
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. For the life of me I can't figure out why Atari didn't just use the SMS pinout for the 2 button 7800 controller. Seems like a lot of trouble just to make sure both buttons fire in 2600 mode. I know Atari 2600, C64 and SMS are all basically the same. I'm not sure what other systems used the same standard. It looks like the Atari 8 bit and ST computers use the same port. It looks like the Amiga does too, but pin 5 is used for a 3rd button, so that wouldn't work.


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