When you get strange characters in the terminal window, the most common reasons are:

- wrong baudrate
- wrong mode (number of data bits etc)
- your embedded system is using characters above 0x7f (127) and using a different character set than the terminal program.

If you get valid characters, but are missing a lot of characters, then you may insert new characters in the transmit register without checking if the UART is ready for more data.

In some few cases, it may be caused by very heavy noise on data or ground. Just make sure that the RS232 transceiver is designed for the same supply voltage that the processor is using.