cascading VDPs

Par st1mpy

Paladin (900)

Portrait de st1mpy

09-12-2009, 17:32

From reading around about VDPs it is supposed to be designed so that it can be cascaded (connect multiple VDPs together), what I want to know is, what is the use for such a thing?
(I know of the VDU project, but if you are cascading TMS9918A what can it do/be used for?)

!login ou Inscrivez-vous pour poster

Par wolf_

Ambassador_ (10088)

Portrait de wolf_

09-12-2009, 18:14

- More sprites
- Separate tiles for backgrounds ( = more tiles)
- Redefining tiles to suggest smooth movement, without interference of other tiles. With stacked VDP's it would be a lot more easy to create that Outrun idea PaulBrk was toying with, some weeks/months ago.

Par Leo

Paragon (1236)

Portrait de Leo

09-12-2009, 18:42

if vdp' clocks are in phase you may merge their digital output on color bus with some and/or/imp/timp operator , to create dual plne just like vsu.

Par st1mpy

Paladin (900)

Portrait de st1mpy

09-12-2009, 19:52

OK thanks. So the next question is. The above is possible because the VDP can take an output from one as an input to another (?)
In a normal MSX, is there a way to input into this input pin(?)
Does that involve a hardware modification (rewiring) / software (BIOS modification of a complete reflash of the BIOS ROM, or altering values in software?)

I guess I should look at the data sheet, but I probably wouldn't understand it.

Par Leo

Paragon (1236)

Portrait de Leo

09-12-2009, 22:51

I am not specialist but vdp can be "slave" for signals like hsync / vsync : thi is usually called "genlock".
So with one "vdp master" and other "slave" , they receive video sync but not video signal itself, you can have each vdp outputting its own 8 bit color bus and then you can combi each 8 bit bus and then use a DAC to drive RGB output.
I believe it is more complicated and this is just principle : VDp's have to be "sync'ed" + vdp have 8 bit color bus output .

color bus direction can be input , this is used for digitize , and i would not use bus input for anything else but digitizing

Par RetroTechie

Paragon (1563)

Portrait de RetroTechie

09-12-2009, 23:08

I believe it is more complicated and this is just principle : VDp's have to be "sync'ed"
If I'm not mistaken, this is done by varying the VDP's clock frequency, within +/- a few percent of nominal. A small enough range to stay within spec and produce stable screen output. Thus it runs a bit faster or slower, until it 'lines up' with external sync (which in this case would be coming from 2nd VDP). When this lining up has occurred (=genlock), the VDP and external source are doing the same things at exact same time, and then it is -relatively- easy to eg. switch between signals (for superimpose).

For multiple VDP's there's little point - easier to just take a faster / more powerful one. For genlock / superimpose / digitize the amount of electronics required is the problem (hence so few MSX machines that can do this), whether the VDP supports it or not is less important.

Par MäSäXi

Paragon (1884)

Portrait de MäSäXi

10-12-2009, 14:29

Does anyone know any old or new project where this TMS9918A vdp stacking is done? They have done it in Texas Instruments laboratories, surely, Wink but any other projects after that?

Though how many 80s companies started to yell around "WE HAVE STACKED TMS9918A VDPs FOR OUR BUSINESSES!! GREAT, ISN´T IT!?!?!?" Tongue Maybe that´s why we don´t know any examples.... Wink