I have an MMC64 and then on the passthrough port I have put my old TFC3 (mostly to get the reset button, but also the fastloader is nice to have sometimes).

However, there seems to be some kind of strange incompatibilty problems. When I reset, the computer enters the MMC mode, and everything works fine. However if I press 'F7' to exit MMC64, I always get a lock-up. If I instead press shift-F7 then maybe one time of ten I get to the TFC3 menu, where I can then choose quite to basic. Once in BASIC everything works fine.

Any ideas?

I'm not sure since i'm not a HW guy, but depending on the glue logic you put in the pass-thru port (if any) I believe the reset signal causes both carts to "compete" over the DATA and I/O lines, typically producing results like you're describing.

MMC64 should always "win" that battle, since it controls the pass-thru port.

I was never able to get that to work.

Even when disabling the MMC64 BIOS and saving config to flash, it won't start FC3.
Pressing F7 "ought to be" the equivalent of doing that.

Why this doesn't work, smells a lot like a simple MMC64 BIOS code problem, which should be fixable.

Mmm.. yep. Thinking more about it, this actually sounds like a bug in the MMC64. Maybe Oliver A or Sasq could shed some light on this...

A bit late, but it's worth mentioning that FC3 doesn't work with most (if with any at all) port expanders too. This problem applies both to original FC3 as well as pirate copies of it.