>I did not work out how to get the shift clock running continuously and smoothly. Even with attempts at precise timing for shift register reloading... reloading the shift register incurs a delay before shifting resumes.
This was in fact a hardware bug http://forum.6502.org/viewtopic.php?t=342#p2310 , rather famously the reason for C64 having cripplingly slow (300 byte/s !!) Floppy drive communication implemented thru bitbanging in software despite using fixed 6526 CIA. Previous VIC-20 used 6522 and floppy drives had to stay compatible https://en.wikipedia.org/wiki/Fast_loader
Dropping this compatibility and using 6526 CIA hardware shift register results in C128+1571 combo reaching ~5KB/s
>Unfortunately, I was unable to get it to work.
Author blamed this on:
>I did not work out how to get the shift clock running continuously and smoothly. Even with attempts at precise timing for shift register reloading... reloading the shift register incurs a delay before shifting resumes.
This was in fact a hardware bug http://forum.6502.org/viewtopic.php?t=342#p2310 , rather famously the reason for C64 having cripplingly slow (300 byte/s !!) Floppy drive communication implemented thru bitbanging in software despite using fixed 6526 CIA. Previous VIC-20 used 6522 and floppy drives had to stay compatible https://en.wikipedia.org/wiki/Fast_loader
Dropping this compatibility and using 6526 CIA hardware shift register results in C128+1571 combo reaching ~5KB/s