Thanks Mr. Kumar, let me know your e-mail and I will send you a zipped file, my e-mail is maumana@hotmail.com. This morning I am reading more about this, I have a book translated to spanish, the book is from France, MICROCONTROLEURS 8051, 8052, BY BERNARD ODANT, it explain very good, I think I am not setting correctly the parameters of my AT89C52. I appreciate your help. As I said in others chats, this small program is just to check serial port communication, I have another one (big 4KB) program of my experiment, but I have the same trouble, the P3.1 TXD do not send the information.