During spare time I made a quick design of TTL to RS232 adapter, that works with my AVR development board. This adapter helps to connect TTL (Transistor-Transistor Logic) level signals to RS232 interface. TTL side is a 9-pin female connector, and RS232 sire is a male connector. The unit is powered from the target board using power pins. The RS232 TTL adapter circuit: Put all the in a plastic case and see what I have got: The inside of the box: RS232 adapter in action: Let’s do some test with this RS232 adapter. First of all, write some example routine for AVR microcontroller: Compile this program with WinAVR ad burn to chip. Then download Bray’s terminal or other terminal software open it and select required baud rate and different settings like parity bits and so on. Start terminal and reset the AVR microcontroller: Get the results. Everything is working fine. Download UART AVR project files here. Don’t forget that in this source AVRLIB library is used. So you need to download it to be able to build.