zmtx and zmrz impliement the ZMODEM error correcting protocol used to
send/receive files over a dial-in serial port.  And they do one thing
and only one thing -- transmit files.  They don't do file conversions,
they don't do remote commands, or any of that other stuff.

Note, zmtx-zmrx is not derived from Chuck Forsberg's rzsz package.  
The only restriction is that you will not use this software for
commercial purposes.  (Commercial licenses are available; contact the
author for info.)