I have an Apple "Disk II interface card" from 1978. For some reason, it does not work in my Apple II+. If I insert it, the Apple won't boot at all. Seems like data bus is corrupted.
Funny thing is the card is probably OK. I tested it in an Apple IIe before. I discovered that the reason for data bus corruption in the II+ is that a PROM IC on the interface card is sucking current from the data bus, when it (the PROM) has no power supply (shut off when device is not selected). The PROM seems fine in all other aspects.
I'm starting to believe this card just is not designed for the Apple II+. Is this correct?