Here’s a quick video showing the override I use to run the Mega PC in PC-only diagnostic mode with the Mega Drive ISA card removed. This is particularly useful when I need physical access to the PC motherboard without the Sega hardware and daughterboards sitting over everything.

Normally, simply removing the Mega Drive ISA card leaves me with no video. The front PC/Mega Drive selector is part of a chain that passes through the daughterboard and Mega Drive hardware. Remove that hardware and the normal mode-selection arrangement is no longer complete. The override shown in the video lets me force the machine into the PC-side configuration while I troubleshoot the motherboard.

I’m fairly sure I’ve mentioned this trick in an earlier post, but this time there is actually a video to go with it.

As always, there are more photos in the photo gallery.

Sources