If you own a Xoomfloppy adapteryou probably know there a nice piece of software for this adapter which allows using a 1571 drive to write back G64 images directly to a 5.25" floppy disk.
This is only possible with a 1571 because of the drive's ability to support burst transfer modes that are as fast as if you had a parallel cable.
Now, I have asked Peter Rittwage if it is possible to make a software for the C128 which could dump the data of a G64 file stored on a 1581 drive or even in a 512k REU to a 1571 drive. After all, all 3 components involved do support burst transfer modes too. He said it is impossible and he gave me a nice CPU cycle chart to prove his point.
The problem I have with that is that the CPU inside the 1571 which receives the data is no faster than the C128's own CPU. If the 1571 can receive the data at a certain rate from the Zoom floppy, how come the C128 in FAST mode can't send it just as fast ? Especially if you take a break between each track, prepare the data in the C128's RAM to organise it in a way maximizing transfer speed over the CIA's serial port.
On top of that, I can easily ensure all the CIAs involved are actually 8521 chips.
But... we talking about 1571 in GCR mode as could remember 1581 it's MFM only. So probably different times to byte read generate they are. Today we can buy 512KB static RAM for $1. Special in 1571 (I don't know 1581 never had) we have free CIA ports (for banking additional 512KB static RAM) and can be immediatelly read all disk to RAM. After this no problem with transwer or GCR decode etcetera.
Also I guess here (1581) can be the DMA controller for read block to RAM...