About original Memotech hardware.
Tony Brewer
Posts: 64
Joined: 08 Jan 2014 20:50

Re: NTSC/PAL VDP setup?

Post by Tony Brewer »

TBC wrote:The VDP datasheet says:

Note: Enabling bit 7 (actually bit 0/LSB) in the TMS9928A/9929A causes A-Y and B-Y to go to the sync level only when all planes in front of the pixel under question are transparent.

I have no idea what they mean by this.
A-Y is an error and should be R-Y. More from the datasheet:
The last and lowest priority plane is the External VDP Plane. Its image is defined by the external VDP input pin which allows the TMS9918A to mix the external video signal internal to the chip.

This mixing must occur outside of the chip for the TMS9928A and TMS9929A. This is achieved through the colour dif­ference outputs swinging to a special level not used by the colour difference signals in normal operation. This occurs when bit 7 of Register 0 is set high. External mixing circuitry is required to detect this change in the level of the colour difference signals and then switch from the VDP signals to an external source's signals.
Posts: 3
Joined: 31 May 2020 10:08

Re: NTSC/PAL VDP setup?

Post by cdoty »

I have developed software for all of the TMS-9918 based systems listed in the Wikipedia article, and the M5 is the only one that requires specific handling based on the video standard. I have even modified the Hanimex Pencil BIOS to run on a Colecovision and a SGM, to run Pencil 2 cartridges on the Colecovision without worrying about the video standard. The only side effect is the music run fast, since the Pencil 2 was a PAL system.

The Colecovision does seem to adapt to the video standard though. Location 0x69, of the BIOS, contains the vblank refresh rate.
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