[scarybeast]

That's an interesting story of the 6522 VIA for sure :) I don't think it is what I was hitting though, for a few reasons:

1) "The 6522 has a bug in mode 011, shifting synchronous serial data in under control of external CB1 clock" -- this is not the mode I tried.

2) The bug appears to be intermittent data loss. What I had trouble with seemed deterministic, the chip is just too slow to respond.

3) I wonder if this hardware bug was ever fixed? The BBC Micro doesn't use the MOS 6522 VIA. It tends to use Synertek or Rockwell. It's unclear if those companies were just using the buggy MOS mask under licence, or something else.