Are you sure J. Guy ?
What I´ve been told earlyer from Atmel and what I have experienced it does echo every byte. ( Not only " U " )
And I need to get it away because I would like better performance on my Half Duplex Solution.

And to Oleg. Yes it is T89C51RD2 ( The Temic version )

I have also tried to disassemble the bootloader hexfile with the disassemler found on this page, but it seems like it doesn´t get correct asm code. Because when I compile it it looks different from the original hexfile even without changes in the asm code.

Cheers.

/ Jimmie