The definitive link from a Risc OS machine to a Cambridge Computer Z88 portable.
It transfers single files between the two machines using the Import/Export utility on the Z88 for the other end.
Version 2 is more advanced, allowing the whole Z88 memory to be archived and individual files to be restored, this is available from me for £5.00.
Download Z88Link version 1.02 (zip file)
Request information about version 2
I can highly recommend the book Z88 PipeDream: A Dabhand Guide for Z88 users, but then I guess you'd expect that, as I wrote it.