In my project i use a 8051 (exactly C505) and a 16C550 external Uart but it did'nt work. I write a testprogram to check if the Hardware works. In this I send some data to Pc and from PC to MCU -- this works without problems. from this i know: - external Hardware works - IO space is correct -- is the same as i specified in install.a51 Has someone a tip or had have equal problems and know a solution? I produce the mon51.hex with the install.bat 6 F6 80 mon51 works without problems if i use the internal uart. I hope someone can help me to find a solution IO-Space for Uart is 0xE400 - 0xE7FF P32(INT0) is for intrpt - pin used Best regards Frank Keubler