interface FTDI ttyUSB0

Hi, i made a UART AVR microcontroller programmer, and also made a C program on an old computer to interface it. I accessed COM1 via IO memory space address 0x3F8 to 0x3FF but in a notebook without serial COM port, cant do that. I bought a USB -> COM (supposed to be FTDI chip based) but i don't know how to interface it in C or C++. Y tried a Windows based COM communications program to test the FTDI and works OK. But in linux when i send a command char: # echo '0' >> /dev/ ttyUSB0 the red led on my board becomes on indicating command error on the UART interface. And also if i do so from C with an fopen("/dev/ ttyUSB0","rw"); fpuc(0x30,.....); red light flashes again... And no response on getc... no answer... Thank you for your time... ^^