I was fiddling with wd1793 emulation (floppydrive controller) and am a little stuck. When the MSX does a 'read sector', it sends the command to the controller and then starts polling for bytes. Now, I tried feeding 512 bytes to the msx but also 584 bytes (that includes the sector data fields) but all trigger a force interrupt on the msx disk rom extension. Anyone got some pointers to clear this up?

I'm not really expert on this, but reading status register should reset INTRQ

Source: http://msx.retro8bits.com/msxwd1793.html

Ah yes thanks but the problem is not that it triggers an IRQ but that I don't know what disk basic expects: just the plain 512 bytes of the sector? (I also read 256 bytes somewhere) or also the sector header?

ah found it: just the 512 bytes of data, no sector headers or anything

Are you making an emulator? There are existing emulators which also emulate this FDC... maybe it's helpful.
However, it might be a good idea to join an existing emulator project, instead of reinventing the wheel... Unless that's what you're after, of course

Most personal emulator projects (including mine) prioritize the author's fun/learning experience/interest. That includes "reinventing the wheel" cases such as writing chip emulation based on datasheets and reverse engineering instead of other emulator's source code.
Good luck with your emu, flok!

That's what I meant with "unless that's what you're after"

Yes, I know of all the excellent emulators.
Writing it for old time sake.
Back in the days (1994 I think) I wrote an msx emulator in gfa basic. As you can imagine it was deadslow and hardly worked. 2 weeks ago I found this z80 emulation library and though what the heck, let's see how much work it is.
The work in progress can be followed at http://www.vanheusden.com/msx-emul/

Cool, you're that guy who wrote that MSX Emulator for Atari! Your site keeps popping up for me Nice to hear from you. Have fun with your emulator!