Thanks Stew and Pere.
I can't pretend to understand if the FIRQ reset will make a difference I'm afraid. Regarding the fan-out - it's not happening on either DragonMMC or DragonDrive, so not sure it would be that - unless the CocoSDC is much more complex in design itself of course.
The streaming module is irrelevant really - the machine will glitch before we even get to running that most times. But for info, the streaming module is just some decent quality 256 colour images being streamed directly from the SD card to the V9958's VRAM using low-level CocoSDC commands.
Re: the WordPak 2+ - I'd need to re-test all of that, as haven't used it in a while. Need to dig out the DSUB <-> SCART as well. Won't be today.
As Pere says, the Y-splitter is literally that - connecting the expansion port to two identical sockets. It's the Oojamaflip. https://www.dragonplus-electronics.co.u ... ojamaflip/
My write-up there suggests that I was using CocoSDC with it, but I'd need to double check. I did get the special OS9 running on there - and that would have either been DragonDrive or CocoSDC.
The DMSX2+ board is currently smoothed with about 4,000uF across 4 electrolytics - mainly to dampen the noise that was coming from the YM2413. Even the original FM-PAC etc for the MSX machines had some massive caps for that purpose. The DMSX2+ can be powered using the Dragon's internal supply (if it's been uprated) or an external +5V supply. The issue has always been the same whichever way it is powered.