Is this a true serial port, or is it Serial Over USB ? If it is Serial Over USB is the destination internally converting the USB stream into something it is stuffing into an RS232 or TTL port (so the end code believes it is reading from RS232 or TTL but there happens to be a media convertor in the middle), or is the endpoint making the data available more directly to the destination (for example DMA or through device registers that have access to the memory) ?
If I recall correctly, some of the arduino series have implemented Serial Over USB effectively as a media convertor, with the bytes being pushed into an RS232 or TTL device; I think I have encounted the occasional OEM-type board (System on a Chip or DSP devices or programmable HDL type devices) that used the same kind of technique to provide the USB interface.
Any case in which the data has to pass through a true RS232 or true TTL serial device, risks data overrun if run at a higher baud rate than can be reliably processed. The cases in which there is a more direct connection between the USB controller and the remote CPU are more reliable for Serial Over USB.
