V9978... what could it have been?

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By tvalenca

Paladin (728)

tvalenca's picture

03-06-2019, 19:33

madscient wrote:

V9990 has been developed based on V9978 specification, but it was not intended to use in MSX.

The original plan of VDP upgrading at the time of MSX2 standard released was implementing multiple VDPs.
Color Bus, included in V99x8 was prepared for that functionality.

IMHO, VDP improvement should be overlaid on V99x8 via its color bus. And if it is that way, any functions will be allowed.
In fact, external palette extension using V99C37 has been released for FS-5000/5500.
We can connect any video device through the color bus, like SMS VDP, Genesis VDP, and V9990 of course.

The important thing is that V99x8 itself needs to be installed for V99x8 compatibility.

Now I am curious: V9990 via V9958 Color Bus? I'll take a look on that!

By madscient

Master (179)

madscient's picture

12-06-2019, 09:18

Also V9990 has color bus function (but I have not confirmed whether if it can be connected to V99x8 color bus)
It is not necessarily to use color bus, also can be using RGB and TS(transparent strobe) signals for genlocking as usually.
Although using color bus allows possibility that VRAM data can be exchanged directly between VDPs without CPU intervention, it will also require very complex circuits and controls.
I think that even simple and generic video overlaying (using TS signal) is effective enough.

By tvalenca

Paladin (728)

tvalenca's picture

12-06-2019, 15:44

madscient wrote:

Also V9990 has color bus function (but I have not confirmed whether if it can be connected to V99x8 color bus)
It is not necessarily to use color bus, also can be using RGB and TS(transparent strobe) signals for genlocking as usually.
Although using color bus allows possibility that VRAM data can be exchanged directly between VDPs without CPU intervention, it will also require very complex circuits and controls.
I think that even simple and generic video overlaying (using TS signal) is effective enough.

If you drive both VDP's using the same 21.477MHz clock signal and connect V9990's /HRESET and /VRESET pins to V99x8 HSYNC and VSYNC signals both VDP's become genlocked to each other, without aditional circuitry. This become crystal clear when you read V9938 manual. the last missing part is to combine both VDP's RGB out using a switch controlled by V99x8 /Ys signal.It's actually a piece of cake to design this because of superimpose capabilities of both chips, but never implemented because it is potentially dangerous to mess with V99x8 pins on any MSX main board.

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