Hi Peter thanks for answer me. This program is just a super small program to test send characaters by P3.1 (TXD) of 80C52. I am doing a big program to do another experiment, but my trouble right now is I can not see any wave form in TXD pin when use this small program test. When I run my simulation on FRANKLIN SOFTWARE it runs ok, but I never see nothing on TXD. Two days ago I found some problem at my circuit there was two wires in wrong positions. I can see wave forms in P0 & P2, then I now my program is running, but I dont know why I can not see anything wave form at P3.1 TXD pin. Could you know why ?