Modding the Snes so the Tototek GBA-SNES adapter does not need separate AV cable.

Discussion in 'Modding and Hacking - Consoles and Electronics' started by MaxWar, Sep 15, 2013.

  1. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Here is the Final form of the S-video version of the Mod:

    *This does not work on all tvs, on some tvs you will need to disconnect the S-video cable to get a composite image from the adapter.

    [​IMG]

    This is a video Demo.



    A picture of my install. Not the prettiest, cleanest job but everything is sturdy ( parts are held down with dabs of hot glue). The way I did the install, It all fits on the bottom of the SNES, motherboard but you need to get rid of the bottom RF shield.

    [​IMG]

    Here is a different schematic, that works with composite only. The only advantage over the S-video version is that it works with all TVs.
    [​IMG]



    *Original first post.
     
    Last edited by a moderator: May 25, 2015
  2. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    Since you're using that amp as a kind of crude switch, I'd consider a MOSFET rather than a BJT.

    As for switching, something like this? (Click here for bigger view)

    [​IMG]

    You just need some source of Vcc that's only present when the adaptor is plugged in.
     
    Last edited: Sep 16, 2013
  3. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    The way I saw it, my thought was simply to make a buffer between the SNES and adapter. I just do not understand why my simple circuit screws up the SNES composite signal.
    But that is interesting, Ill try to read some more about MOSFET. Im currently lacking knowledge about how a MOSFET would be better.



    I really like the logic gate idea. The more I think about it, the more I think such a switching approach would be better than my transistor amp approach.
    However I do not know how I would be able to get a VCC from only when the adapter is present. The adapter connects to the cart slot In a way that I expect is not different than a regular cartridge from the SNES point of view. Maybe it is possible but in any case I think that would require some serious examination of how the adapter interfaces with the SNES.

    However there might be something to be done with the EXPAND line! It is basically useless in the system, probably was intented to let the cart know when a device is connected to the SNES expansion port. AFAIK this is 100% unused feature. So the EXPAND line is just... there. The line is normally high from a pull up resistor but you could replace it by a pull down resistor instead, which would make it a solid 0 during normal SNES uses. When the adapter is present this 0 would be replaced by the actual composite signal. All that would be needed would be some circuit to detect the presence of the analog signal and activate the Signal path switch. Sounds simple in theory but I have no idea of what parts would be needed for this :p
     
  4. TriMesh

    TriMesh Site Supporter 2013-2017

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    My guess is that as soon as the SNES PPU comes out of reset it starts to generate sync signals, even if it doesn't have anything to display - basically, that blank screen is actually perfectly valid video except that that it contains black during the whole of active line time.

    So when you common that up with another video signal out of the converter you end up with a signal that has two sets of sync pulses with a random phase relationship between them - something that no TV is going to be able to make any sense of.

    I think your best approach might be to find some way of disabling the SNES video encoder to stop the sync from getting through - I think I have a copy of the chip datasheet somewhere, but I couldn't find it just now.
     
  5. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    This seems like a valid approach but there is a common point with the logic gate switching idea in that the system will need to differentiate between a normal SNES cart and the GBA adapter to decide whether the encoder is disabled or not.

    So far the only idea I have about how to do this revolves about detecting when there is composite signal in the /EXPAND line. There might be something to be done with the other pins like the /CART one but I do not know enough about the SNES cart interface.
     
  6. Bearking

    Bearking Konsolkongen

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    Interesting product. Possible to select 1:1 output where it doesn't scale to full size? Does it output in 480i or 240p?
     
  7. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    There is no user accessible control to deactivate the scaling, it is always scaled to full screen. If there is a ways to deactivate it by hacking the hardware I do not know.
    As to the output type, I do not know! Good question actually. Do you know of an easy and reliable way to tell? Interlaced video can look slightly shaky on my CRT tv normally. I have not observed this on the adapter but since it is composite and everything is somewhat blurred out it might be hard to tell.
     
  8. Bearking

    Bearking Konsolkongen

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    Not without using something like an XRGB, it will tell you if it's one or the other. You should be able to tell from your CRT too, but since it's composite it might be so blurry that it's hard to tell :)
     
  9. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    The benefit with using a MOSFET is that the source-drain impedance in the off state is very high, much higher than in a BJT.

    I'm still pondering how you could detect whether the adaptor is inserted or not... Hmm....
     
  10. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    I looked at the SNES composite signal with my oscilloscope. I dont see anything being output when the SNES is powered with no cart in it. Just a flat line. When there is a SNES cart, hence there is a signal, the waveform seems to keep under 1v peak to peak and modulates with what's happening on the screen, as you would expect. It makes some funny patterns at times :)

    The interesting part is when I looked at the composite out of the SNES when the adapter was present. Despite being no image on the screen, I got a steady high freqency waveform on the scope, for some reason I could not lock down on it but it was there. I guess that is the sync only! So your theory appears correct. At least in part. The encoder seems off with no cart present. But the GBA adapter presence makes the encoder output sync , even though the encoder in not really being used.
     
    Last edited: Sep 17, 2013
  11. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    When no cartridge is inserted, the CIC lockout is keeping the system in a reset state. Not knowing a whole lot about the SNES' architecture, my guess is that when the system is in the reset state, the video encoder is inactive.

    To test, probe the video signal, and see what happens when you push the reset button.
     
  12. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    It does behave as you expected. When reset is held, output of composite is flat no matter what is in the cart slot.

    I was thinking some more about the whole deal, all that would be needed is a system that reacts to the presence of composite out of the cart port and latches a High state output. Then we could do whatever we want with it. Like triggering an electronic switch or deactivate the encoder, as Trimesh suggested. Im sure there is a simple way to achieve this.

    Composite peaks at less than 1v it seems, not enough to trigger your typical 5v logic but could you just amplify this and use this to trigger a latching logic gate? That would pretty much solve this topic i think.
    Or alternatively use a logic system that works with low voltage?
     
  13. Gamesquest1

    Gamesquest1 <B>Site Supporter 2014</B>

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    Hi I was thinking about this recently,
    im not really much good with DIY circuits I can follow instructions but making them up my self is a no go, the solution I would probably try would be having 2 tabs inside the connector one which grounds the original snes av out and the other connects the output source from the device to the snes output, not too sure how good this would work but that's what a circuit designer idiot like me would do :)

    im sure there would be a better more technical way to do it but that's the best answer you will get from me :p


    Edit actuall I had forgot about the carts that use the extra connectors as ground if I remember correctly suppose the way I was thinking would cause any carts that have the extra pins to ground the original snes av aswell suppose you could always add your own flat tab inside that does the grounding separate from the original connectors but who knows that's just an idea that wouldn't require any extra parts really just wiring :)
     
    Last edited: Sep 17, 2013
  14. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Those two tabs are used by stuff like SFX games and the game boy player and this Tototek GBA adapter uses them too for whatever things they do. It contains some address lines and I dont think you could safely Hijack any of them without losing some functionality. As far as I know the only truly usesless connection in those tabs is the EXPAND one. This is why it is used to send composite video in this context. It was pretty convenient from Nintendo to leave us a useless connection in the cart slot, I think expecting a second one would be too much to ask for :p

    There is no room in the connector to add a new tab but you just gave me a somewhat funny idea. Right on the side of the connector inside the SNES there is a little free space where a hidden switch could be installed. You could glue a smal block of plastic on the GBA adapter that would press that switch when It is inserted in the SNES. A regular cart without that additionnal block would not activate the switch.
    This is pretty crude, but it would work. I think this is how a Goblin engineer would do it :p

    This being said, I would like to avoid this method, It kind of breaks the magic :p
     
  15. Lum

    Lum Officer at Arms

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    Interesting... Never heard of these pins.
    If I'm not mistaken, being an RGB based console after all, wouldn't the SNES need a video decoder for this to work right?
     
  16. sparksterz

    sparksterz Spirited Member

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    This was my main area of confusion as well unless the plan is to bypass the entire encoder and go straight to output.
     
  17. Gamesquest1

    Gamesquest1 <B>Site Supporter 2014</B>

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    yeah we'll that's the kind of thing I was suggesting, guess I'm a goblin :p yeah it's not the most technically perfect but its simple and still pretty neat, you could easily hide the switch just in the gap so it's not really visible unless your looking for it

    in sure someone will come up with something more technically superior but I am a mere goblin XD
     
  18. MaxWar

    MaxWar <B>Site Supporter 2013</B>

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    Well, I was not planning to use the SNES encoder. This would involve decoding the composite back to RGB only to have it re-encoded again. So yes, im aiming straight at the output. Of course this means the adapter would only work when the SNES is connected with composite but you would still probably get a better image than you would by getting it through a second encoder.

    Haha, hope you are not offended by the goblin reference. I did not mean to compare you to a goblin. I was thinking about the goblin engineers in the Warcraft games and the way they build things


    OK. So at this point I will work on a specific goal. I want a binary logic operated switch to select between two composite signals. I will worry about the rest after. Retroswim, I know you built this huge fully electronic video switcher. Any recommendation as to what part to choose?
     
    Last edited: Sep 17, 2013
  19. Gamesquest1

    Gamesquest1 <B>Site Supporter 2014</B>

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    Haha nah I got the reference no offence taken, rather be a goblin than a gnome anyways :p

    good luck anyways shame they could design them to just work in the first place XD

    ps yeah I know all the technical reasons it wouldn't work, still would of be good if it could of :)
     
  20. RetroSwim

    RetroSwim <B>Site Supporter 2013</B><BR><B>Site Supporter 20

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    In the diagram I posted earlier, I used NAND logic gates. You'd just use a 7400 quad 2-input NAND IC for that.

    To switch the signal, I used a bus switch IC SN74CBT3244, but since you only need to switch two lines on to a single-line bus, a pair of N-channel MOSFETs should do, like the 2N7000.

    Disclaimer: I haven't personally used the 2N7000 for switching composite video before, but the bus-switch IC I used is essentially 8 n-channel MOSFETs in one package, so I don't see why it wouldn't work. You might be able to find another n-channel MOSFET with less source-drain impedance, perhaps.
     

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