Years and years ago, I was following a discussion about Commodore’s 1541 drive and its programmability. Back then somebody claimed that two 1541 drives can be instructed to communicate to each other, e.g. to copy a file from one to another, without intervention of a host computer, beyond the initial setup stage.
Now, I can certainly envisage this being possible with custom drive code running in at least one drive, however, using exclusively the standard serial protocol and ROM code it seems unlikely to me that there’s any truth to such claim.
The reason I am wondering is that I plan to build a new command-line application called iechost-copy that will be able to copy files from one drive to the host computer, and the other way around, or from one drive to another, as long as they use different device numbers. The latter scenario seems to always involve transmission to a host first, whether a file is first fully read in and then written to, or read and written byte-by-byte.
If anybody has any information about the two-drive file copy without host intervention claim, feel free to comment! I don’t require working code, but just a summary of how that would be done in practice.