@hipsterelectron @neffo Here’s the gist for connecting the arduino to the Beeb keyboard I/f https://gist.github.com/8bitkick/b52d1933dd8a8965960e3ede01781f3d#file-beeb-keyboard-ino
I used a logic analyzer to find the signal timing. After the fact, I found the code responsible in the BBC Micro OS ROM… cycle count checks out https://tobylobster.github.io/mos/mos/S-s17.html#SP34