OK I've replaced all occurrence such as INT or UNSIGNED INT by int16_t and uint16_t as found in my MEGA running SerPro protocol so it will more portable. Runs fine with my MEGA board but still fails with DUE even though compiles & downloads OK on DUE.
Unless I make an error, it is obvious to me that the Serial functionS set or family (i.e. read(), write(), event(),...) do not work on DUE if called inside an IDE, at least at 115200 bauds !