Enter Rhea

gdemu 38 Comments

So this is Rhea V1, the Saturn ODE prototype, playing Yello – Pocket Universe on my TV:

Rhea V1

At the moment playing audio is all it does as I haven’t gotten around to implement protection emulation yet. I’m in no hurry, this is a prototype and it’s main purpose is to test how stuff works, what I can and cannot do.

Saturns use 2 different drive types: 20- and 21-pin ones. Even though it looks like a small difference, the protocol is actually quite different for both models. This prototype has only 20-pin connector but the design was meant to handle both formats with just once MCU and no external logic chips like FPGAs or PLDs. If it works that way at all that is, but so far it’s very promising.

Just so that we are clear on this: V1 is not for sale, and in general not quite fully functional yet. All questions about the final product, it’s timing and price, will be ignored. It’ll come when it’s ready.

38 thoughts on “Enter Rhea”

  1. That’s AWESOME!! Glad to see you making real progress on this project.

  3. Very Cool! So you’re intending to do an Optical Drive Emulator along the same lines as the GDEMU? Have you thought about an MPEG card solution like some others have been working on? In any case I’m excited to see progress!

    • I prefer direct drive replacement. This leaves the MPEG bay free for whatever people might have in there, and is actually simpler for me. Also, the connector in there is huge and would be expensive and PITA to solder down.

      • Awesome thanks for the reply, I’ll definately be in for one of these when you decide to release 🙂 and another GDEMU I’m loving the one I’ve got right now, playing some THPS and THPS 2 with insta-load times brings back good memories 🙂

  4. Awesome job.Could you explain please, the setup showed on picture?Rhea is crearly the board with inserted SD and what about the other ones?Thanks.

    • This is mostly dev stuff. USB UART, JTAG and voltage level translator for FPGA LA (not connected). The final setup will only have one PCB on the ribbon cable connected to the mainboard. And maybe one more wire to provide +5V.

  5. There are actually 3 different 21pin drives too, they differ in the ribbon pinout for sure, I wouldn’t know about the protocol though. The last drive type has different clock signals in the ribbon, which is the reason why all modchip guides are retarded to this day.

    And then there are the alternate drives they used – one Hitachi (20pin), and three Sanyo (one for the 2nd 21pin type, and two for the 3rd, last 21pin type).

    And excellent choice on Pocket Universe, easily the best Yello album by far, oddly enough since it is so different from their usual performance.

    • I admit I know little about 21-pin types. All my Saturns are 20-pin ones. I’ve read that the clock supplied by the drive to the sound subsystem is different but I should be able to handle most popular values derived from 33.8MHz.

      As for Yello, yes, it’s different. It’s like they decided to troll the younger generation and show them how electronic music should be done 🙂

      • The first 21pin drive was still using a Hitachi controller.
        The second one switched to a Mitsubishi one. They also duplicated one of the data clocks into what was previously the access led pin.
        The third one omits the onboard 33MHz crystal, and has the 8MHz clock output in the ribbon exchanged into a 33MHz clock INPUT (the drive now receives the clock from the motherboard). But otherwise is the same as the second type, as far as I know.

        I haven’t tested if they are compatible with each other should these changes get reversed, I do recall that one of them works in another if a modchip is tucked inbetween (but that really doesn’t say anything).

        Personally, I think the changes were specifically made to thwart modchip compatibility.

        How is the protocol different for 20 and 21pin drives?

      • 20-pin uses separate serial outputs for CDDA and data. AFAIK 21-pin has that muxed into one. Which is better as I have only 1 fast serial controller on the MCU. So far I’ve learned that you can’t just wire those two together on 20-pin console, if you do you will hear all the data burts as CDDA noise. Hopefully simple separation of the audio data line will fix that…

      • The deal with the clocks is a bit random – the first series of 21 pin drives used an on-board xtal and generated 8.4672MHz that was fed back to the CD controller in the Saturn.
        The next series removed the xtal from the drive board, but kept the 8.4672Mhz clock – it was just sourced from the other end.
        The next revision may run on either 33.8688MHz or 16.9344MHz (still on pin 1) depending on the drive type – on of the pin on the connector (16?) is use to select which, and is grounded on the JVC drives and floating on the Sanyo ones.

        The other general rule is that if the drive PCB has a white line around the edge, then it needs an external clock.

  6. Awesome work DK – really excited about this project 🙂

  8. Looking forward to more details. It will be very important to sort out the various 20/21pin variants – there are quite a few about. There are some people on assemblergames that have quite a lot of knowledge about this stuff that may be useful for beta testing – there was a recent thread about a ‘universal mod chip’ which resulted in a lot of testing and knowledge thrown about.

    Actually, that brings up a follow-on point: will this replacement drive also encompass the functionality of a modchip? As I understand it, all the regioning is handled by the drive, so I suppose you must have some plan regarding that, one way or another…

    • No, region is always handled by the console itself. You will still need BIOS or chip mod, or simply use AR to load games.

      • Oh, of course. The drive does the copy protection (hence the mod chip), which will obviously be handled by your device, and the regioning is separate. My mistake 🙂

  10. Are you sure about that separation? According to my notes and the spec sheet, the 20 and 21pin (first ver) drives are all wired to the exact same stuff on the Saturn side, the order is just shuffled for the last half of pins, and the CD Access led is added as an extra pin. They are still connected to the YGR019 pin 122 (subcode), 124 (audio), and 130 (data), as inputs. Outputs are all different pins on the drive side as well, not the same signal. Only the clock signal is the same thing for both audio and data input.
    The only oddity is that the cdda pin and the data clock pin both go through a logical AND gate (VHC08) before going into the YGR019. I’m not sure offhand if the 20pin machines have that as well.

    I did not ever tried probing what is on those signals though. But it would look odd if they were outputting the same stuff on 2 different pins.

    • As I’ve said, I don’t have 21-pin model. I’m basing this on somewhat suspicious pinout I found for the modchips. Say, could we maybe continue this via email? I might have a few questions for you 🙂

      • Sure, go ahead. I’ve been collecting saturns with different boards exactly so I can provide help with this kind of stuff.

      • Cool. I don’t have your email though, could you maybe ping me (dknute at gmail) to get this started?

  12. Love the name, wonder how many people know what Rhea is though? He he!

  13. This is AWESOME! I hope you can make it dknut! You are the man 🙂 Your GDemu works great 😉 I love it. Wish you good luck with Rhea!

      • Oh. I thought it was named after the bird, because it’s not ready for launch (it cannot ‘fly’). The moon thing makes more sense, though.

  15. the action reaply will be needed for the region but will your device give the 4mb of extra ram…., its just a question since i already own a ar cart

    continue your great work

  20. Hi,
    Awesome news! I can’t wait to have one for my Saturn! Let’s know when you open the preoders!

  21. May I ask what hardware you’re using for the board (I know you said it’s not a FPGA)?

    • A) I don’t have VCD card but I don’t see why it wouldn’t work. I aim for perfect compatibility.
      B) Just an MCU plus some 5V level shifters. Works so far 🙂

  22. Hi! Are you still accepting orders? Looked everywhere on the site and can’t find or see anything. Thanks for your hard work!

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