Zero dB schrieb:
It did echo my characters with local echo off, so it definitely madethe round trip through the card.
That means cabling and level shifters work. Your first test (with IN#2) proved that communication to the 6551 is possible as well. So now I would swap crystal or UART. Swapping the 6551 is easy, but the crystal requires soldering. You can try to connect a known-good part in parallel, this works in 90% of the time.
In case you have a contact problem in the slot connector, as mentioned elsewhere, the data read from the firmware EPROM would be corrupt as well. If both $C200 and $C800 ranges look normal (the listing is in the manual), your address and data lines are fine.
With a defective or missing firmware ROM ADT 1.21 will still work, because this sofware does not use the PR#2/IN#2 routines.
Patrick