HI Ahmed, I test your program at simulator (FRANKLIN SOFTWARE) and it runs ok, when I write something in INPUT that write at BUFFER. The point is, I dont see any wave form at P3.1 (TXD) of 80C52, it's the same behaivor than the others programs that I tested. Why I can not see nothing of signals at P3.0 if I can see 0000000 & 1111111 at P0 & P2 ??????