Hi, I'm french so excuse me for my English. I want to communicate with a PC and a 80c552 card made by myself, this with a serial link. I have no INT and just an emission of an octet from the 80c552 card to the PC. My problem is that the PC receives values but not the good one. In fact only FF is received correctly. I have made a communication between two PC so my program on the PC is ok. So the problem comes from the program loaded on the 80c552 and written with keil c51. I use the mode 1 of serial IO with SM1=1 and I have tried several speed from 2400 to 20000 bauds but nothing changed. I have no parity and just an octet with a start bit and a stop bit. Thanks for your help, please answer in english (your french is not so good...), and tell me if there is a better forum for this kind of problem. Florian