MD/Genny auto-switching TMSS bypass mod

Discussion in 'Modding and Hacking - Consoles and Electronics' started by TmEE, Aug 25, 2008.

  1. retro

    retro Resigned from mod duty 15 March 2018

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    SEbA? Why not have it say 5E9A?
     
  2. drakon

    drakon Gutsy Member

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    lcd 9 doesn't have a line at the bottom, so it would look like a backwards p
     
  3. Alchy

    Alchy Illustrious Member

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    Otherwise known as q?
     
  4. TmEE

    TmEE Peppy Member

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    "6" is close enough to "G" IMO...
     
  5. retro

    retro Resigned from mod duty 15 March 2018

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    Last edited: Jan 22, 2009
  6. TmEE

    TmEE Peppy Member

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    VFDs are much more jawusum than LED displays :3
     
  7. drakon

    drakon Gutsy Member

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    sorry I draw my q's with a line at the bottom
     
  8. retro

    retro Resigned from mod duty 15 March 2018

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    Fine... use an alphanumeric VFD, then! :rolleyes:
     
  9. TmEE

    TmEE Peppy Member

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    I need to cough up many dorrals if I want alpha-numberic VFDs.... the fancy displays cost even 500USD D:
     
  10. drakon

    drakon Gutsy Member

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    yikes....how much did these vfds cost you?
     
  11. TmEE

    TmEE Peppy Member

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    Those 4 tubes cost nothing... maybe a cut in a finger when I dug that calculator I stole the VFDs off from a dumpster... people throw all kinds kinds of good stuff away.
     
  12. Pasky13

    Pasky13 Member

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    Anyone got a picture of the original schematic which has now disappeared from the first post for the auto-switching TMSS bypass? Thanks.
     
  13. horstenss

    horstenss Active Member

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    Information is still missing. Can someone help?
     
  14. TmEE

    TmEE Peppy Member

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    Fixed the image in first post.
    All files that I had in fileden are archived at my site, link in my sig to that archive :p
     
  15. Helder

    Helder Site Supporter 2014,2015

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    Anyway you can update that schematic using some modern tools like EagleCad or Kicad? Having a hard time figuring what pins are what.
     
    Last edited: Sep 24, 2014
  16. horstenss

    horstenss Active Member

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    Yea, I second this. Or even better, a small article on the theory behind the mod :)
    Ideally tmss should be removable from all Mega Drives with a mod as simple as possible.
    There won't be a need to turn it back on either as it's entirely useless today.
     
  17. TriMesh

    TriMesh Site Supporter 2013-2017

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    I can give you the basic idea behind it.

    Normally when you plug a cart in it grounds pin B32 of the cart connector - this is wired to the ASIC and when it's grounded the TMSS ROM gets paged in. So if you just cut this line it disables TMSS - but it also disables the ROM CS/ line fed to the cartridge ROM, so the game won't run. To get around this, ROM CS/ (pin B17) is disconnected from the ASIC and wired directly to A23 on the CPU address bus. This maps the ROM into the lower half of the CPU address space, At this point, the ASIC still doesn't know the cart is plugged in, but the CPU can see valid instructions and will run the cart anyway.

    This is obviously a very simple modification (2 trace cuts and one piece of wire), and it does effectively disable TMSS - but it also breaks compatibility with the various Mega Drive/Genesis addons. The 32X and the power base converter won't work at all, and the Sega CD will work, but you can't use cartridges when it's plugged in (and in fact run the risk of damaging the cart, Sega CD or both through bus contention).

    You can fix the 32X and PBC compatibility by checking the M3/ line (pin B30) - if it's low, you have an accessory connected and you should renable CART IN/ and connect ROM CS/ back to the ASIC output pin. From a quick look at the schematic, this seems to be exactly what the mod in the first post is doing.

    Personally, I would be inclined to use an analog switch chip to handle this, since it would be a lot simpler

    Try this - note that it hasn't been tested and might not work, but it looks reasonable, and it's just a single chip.
    TMSS mod.jpg
     
    Last edited: Dec 2, 2014
  18. Helder

    Helder Site Supporter 2014,2015

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    As always thanks Trimesh for the info and schematic, I made a simple Dip package board that costs $4.40 for 3 on OSHPark for anyone to try and a look up on Ebay bring up the IC for ridiculously cheap prices.

    https://oshpark.com/shared_projects/wsgnUwWk


    [​IMG]
     
  19. TriMesh

    TriMesh Site Supporter 2013-2017

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    Ah, one thing I should make clear - if you are using a Sega CD / Mega CD then you need to add a switch to disable the mod, since otherwise the CD unit tries to boot even with a cartridge plugged in. This is pretty easy to install, though - just put a SPDT switch in the line between the B30 input on the board and connector B30 (with the common connected to the board) and wire the other side of the switch to ground. Switching the switch over to ground will disable the mod.
     
  20. Helder

    Helder Site Supporter 2014,2015

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    Anyway to have that also switchless maybe using another ic in the schematic?
     

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