Okay, I found a version of XMODEM that assembles and works properly.
It seems that the one provided with the SDcard images is V2.2 (7 Oct 2014 @ 8KB), while the one I found is V2.4 (4 August 2016 @ 4KB). I've assembled it with ASMX (Bruce Tomlin), uploaded it and used it with the Z80MC XMODEM.CFG supplied by Josh.
I've attached the source code to this message.
Peace and blessings.
