Great video. Gives me a clear piece of the puzzle. Wanting to do a 512 x 8 switcher with a 39040. A lot of old car ECUs used 27256 and 27512 eproms. Have to learn how to design the board now.
One thing that wasn't clear, was you need to remove the jumpers for any address lines you plan to pass through ("D" mode), or you could short the address lines in question. It's shown clearly in the video, but wasn't described. Or maybe I missed it. But I was able to use a ST 27256 EPROM with two ROM images using this switcher once I found my mistake and corrected it.
