OK, as promised here is the source and the binary and even a .DO image
with both binaries.
http://alexfreed.com/FPGApple/xmodem.zip
The (almost) original version xmod.a requires the transmitted file to
have the destination address in the first 2 bytes. This works fine if
you assemble the source say with ACME and set the output to "cbm" format
that does exactly that - prepends the address in low endian format.
However to work with arbitrary raw binary files I made a new version -
xmod1.a
This one asks for the destination address in hex. 4 digits expected, so
to load to $800 enter "0800".
The SSC is expected to be in slot 2 and is not initialized, so the first
time do "IN#2".
Have fun.
-Alex.