We often use RS232 serial communication in our projects. The USB converters are very convenient. For output, these converters can receive from CMOS levels (0 to 3.3V). For input, the ones we use provide about 6 and a half volts positive and negative. To interface to CMOS 3.3 volt logic, I have created a circuit to drop the positive 6.5 volts down to a little under 3 volts, and also completely block any negative voltage. It works well at 9600 baud.
Update: I tried different baud rates. The maximum normal rate is 921600. The waveform received at 460800 did not have sharp rise and fall times, and was not symmetrical. At 230400, the rise and fall times made less difference and the symmetry was closer. But I wondered if it was my circuit introducing the difference in rise and fall times, as 2.2K resistors do not pull that hard. Nope, it is actually the USB converter which has the bit jitter. Also the differing rise and fall times — their method of creating the negative voltage must have less oomph. I measured bit times of 4.48us for the first bit, 4.28, 4.2, and 4.32 for the next bits. I ran a test overnight and had no communication errors at 230400. That is 24 times faster than 9600.