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Thread: A rather unusual BIOS swapping issue with Sega CD Model 2

  1. #1
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    Default A rather unusual BIOS swapping issue with Sega CD Model 2

    Alright this is going to take a little setup to explain. I've got numerous Sega CDs hanging out around here. My model 1, and three model 2s. One model 2 is the original Funai board and the others are the revised Sony board.

    I recently took the plunge and region modded all three via installing the 2014 custom l_oliviera BIOS to each of them. I went with the Japanese one since it's supposed to have full compatibility. Easy enough to rewrite the chip on the Model 1 and it performs excellently.

    The Funai model 2 also went perfectly. I selected the right model bios and flashed a chip then (after desoldering and installing a little pin board to make things easier on myself in the future) tested and it works fine on both Japanese and US versions of Sonic CD.

    Then I did the same mod on the Sony revision of the model 2. Of those two, only one works, so that's the one I did. The custom BIOS appears to work correctly, except that when starting up Sonic CD, either version, the music is delayed by several seconds until after Sonic's turn and finger wag on the title screen. I noticd this because that turn is supposed to sync up to the very start of both versions of the song.

    Alright, all my thoughts went to issues with the board and the disc drive. The one tool I still don't have is an oscilliscope, so it took some guesswork to try and adjust pots. In the end, I returned each pot back to it's exact starting point, as nothing improved tracking, and frankly other than delays like that, it didn't seem to have issues reading discs anyway. I even did a full cap check and replaced the power regulator with a newer cooler running model.

    Edit: In addition, I swapped the laser array (the whole unit) with the one from my other Sony model Sega CD 2. The same results.

    Then, on a whim, I tried out a different custom BIOS. I grabbed a spare chip and flashed from that same set, picking the US version this time. No delay! The music syncs with Sonic's animation perfectly every time. Alright, maybe the chip was the issue, so I erased and reflashed the exact same chip with the Japanese bios again, and the delay returns. I swapped this SAME chip into the Funai model, and in both US and Japanese BIOS, it syncs up perfectly on that version. I went back and pulled up the 2010 version of the Japanese custom BIOS and installed that. Same delay. Nothing changed in the code causes the issue.

    In other words, I narrowed down the issue and it seems the Japanese BIOS for the Mega CD model 2, the one revision l_oliviera seems to have modded, adds some odd slowdown to the US Sony revision of the Sega CD. Any thoughts? I have to admit even tracking down the custom bios was a nightmare involving trudging up ancient forum threads referencing dead web sites which I then had to stick into archive.org, leading further into a rabbit hole of vanished sites until I finally found a backed up link working somewhere. So, I have no idea if the 2014 release of the modded bios files is even the most current one any more.
    Last edited by Dark Jaguar; 07-03-2022 at 12:36 AM.

  2. #2
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    I could find it possible if the Sony revision was US only and so the BIOS was updated to handle that chipset. Unlike on later hardware (both PC and console), on the Sega CD the main CPU was directly doing all communication with the CD drive chipset, so if a bios does not support the chipset, there would be bugs.

    It ain't like the Saturn where you can mix and match all the boot roms because the CD drive uses a standardized I/O that a select sub-cpu translates into normal commands.

    Quote Originally Posted by Dark Jaguar View Post
    I have to admit even tracking down the custom bios was a nightmare involving trudging up ancient forum threads referencing dead web sites which I then had to stick into archive.org, leading further into a rabbit hole of vanished sites until I finally found a backed up link working somewhere. So, I have no idea if the 2014 release of the modded bios files is even the most current one any more.
    The latest ones I have is the 2014 archive:
    http://segacodes.org/junk/MEGA-CD_Region_Free_ver2.zip

    As far as I know there weren't any newer versions released. But you should ask l_oliviera directly to make sure.

  3. #3
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    Thank you for that added detail! On top of that, I've replicated the issue. I spent this morning repairing my other Sony style Sega CD 2. It had a broken trace in a rather highly compact area but I managed a little wire run. It helped having a fully working model I could continuity test to make sure it was all going where it was supposed to (and I found a little grounding issue I'd caused in the process but fixed that one right up too). Once I confirmed it was working, I went ahead and stuck a 40p socket in so I could easily swap BIOS chips and yep, exact same symptoms that also disappear when using the latest modded US BIOS.

    Thank you for that link! It's good to know there's yet another Sega site I can refer to for certain resources. I hadn't heard of that one, but it confirms I am using the latest revisions after all. I honestly have no clue how to contact L for more details. Slippery fellow ever since that original site went down.

    In any case, now that I'm two for two on Sony boards showing this issue it's safe to say that's what it is.

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