F232 is USB powered, but you need to have a level translator at the output of F232. Tx of f232 has to be converted to RS232 levels, since the other side has a max. Receive pin can be easily opto-ed. But the transmit pin, need to have a supply in line with RS232 levels. Sometimes we d it by making a higher supply from the remote RS232 chip - a common method, if you are in control of the remote max pin.

Here is the circuit. I modified the circuit to replace rs232 chip with MAX232 to the board sold by FTDI guys. FTDI has got circuit + gerbers in their site. I did not use their gerbers. Got a layout made by someone.