Dual V9958 board

Yes, I saw that. But the developer isn't releasing the info, and I have something different in mind. Too bad that Yamaha never published the whole information..

I am actually working on a prototype with this objective. It's just simpler to have two well known VDPs than "force" people to learn how to code to a "new" VDP (V9990).

I'm currently having some issues with the balance between some features I want the board to have and the simplicity to install and use. And I've learned from GFX9000/PowerGraph that's not pratical to have two TVs/monitors hooked to one MSX im most cases.

MSX already has a VDP. Why not make a cartridge with a single VDP and a video input for superimposision with color 0? The ports of external VDP could be 88h~89h as the MSX2 expansion.

Those links don't work.

That is a very good start point, and actually was my starting point. So, anyone could just insert a cartrige and plug 2 cables in it, and even those without any soldering skills could use it. But doing this way you'll always have the external VDP over the internal VDP and may not be possible some effects like on e1000 videos where he superimposed Gradius/Nemesis and Galaga over some very cool movies from DVD using a Sony HB-F900 and HBI-F900.

Sorry to say that, but there isn't a "better solution than superimposition" here, unless you re-do the complete thing (replace the internal VDP with a new improved VDP, like the TI99/4A and ColecoVision folks did with the F18A). You can't run two things on the same I/O port at the same time, or you'll have bus colision on some very important bus signals, if you ignore that you will most likely fry something, I'm sure the Playsoniq team had a massive pain in the ass with this. (External mirroring VDPs could only be write only, you have to rely on internal VDP for reading anything as for interrupt generation! And this is impossible to do on Turbo-R's)

About doubled features, it's not quite the way, because you can't have hardware collision between VDP #1 and VDP #2 sprites as they run completly independent internally (even when superimposed), so you have some overhead to put things working as it were one.

What I'm trying to do is to make things easier to the coders, while adding some power to it other than "parallel-superimposed graphics processing" (without being so overpowered like the last version of VDU, the VSU, which never got released for unknown reasons). It has to be that way, because they'll have to deal with not just a single V9958, but two of them... And some BIOS integrations of course, if you can't play with it in BASIC, it won't be so fun. As we say here in Brazil, let's not spread the vapor, so it won't solidify.

Anyway, at some point I will need input from potential users, (it's not fun to play alone) so feel free to comment if you wish.

the problem with mixing two video signals outputs from two original 9958 is that, you can't make sure it be in sync all the time "about vertical & horizontal position", one of them can drift, if not working with the exact same oscilator cristal, so the idea of an external 9958 to be sync with the internal, is impossible.

also is the problem of it being in two different I/O, if vdp registers on both VDPs aren't setup at the exact same time, it will have unsyncronism from that point. Overal if the vdp registers are changed in about NTSC/PAL mode.

A mix of 9938 + 9958 as secondary is more doable, through the build-in superimpose feature, where one is conducted through the internal.