Just finished measuring the SWIM. The Address bus lines and Data bus lines all connect from the legs of the SWIM chip to the corresponding pins of the PDS.

Thanks for the information about the filter networks! Just tested the it to be on the safe side, its completely ok. Looks like it protects the external port from frying the complete logic board when something is attached wrong like RickNel's case.

Just tried writing to a 1.44mb disk again, that works, reading off the disk mostly works fine...

just the formatting (especially on the 1.44MB HD disks) just causes instant bombing with a different 'bus'-related error before even writing one thing to the disk.

Would changing out the +5V decoupling capacitor C11 make any difference for the stability of attached drives?

...Whats next to test, any ideas?