I found out in my CX5MU the TMS9918A EXTVDP (pin 35) is direct connected to pin 6 on slot 3 (sound module). So the other than a genlock module I can only think in a laserdisc module, since Yamaha produced them as well.
Well it seems it does not exist but what I am saying is that Yamaha had plans for it. Why would they put a ext vdp signal for a sound module port ?
The Yamaha module port isn't exclusive to sound hardware. Yamaha sold modem, word processors, network interfaces and other equipment that uses this module port.
About the VDP output in this port, it was used by some JVC/Victor modules, remember that the first MSX models from Victor were Yamaha OEM. Victor expertise was in video equipment. So, it sold a superimposer (Genlock) and a RGB output module.
So for 9918A machines the slot pins in the wiki page is wrong. They don't have Y, R-Y and B-Y. Instead the pin-6 is the ext-vdp which is input instead of output.
