The data lines are good up to the voltage level of the Vcc line.
So if power from 3.3V, your data out lines will be ~3.3V and any incoming lines should be <=3.3V.
If you power from 5V, same thing: outputsp will be ~5V, and any coming lines should be <=5V.
Thanks for replying. Well that was my point on the one USB to Serial device I have. You cannot jumper it to put out 3.3 or 5 volts but it has a Vcc reading 3 volts and another Vcc that reads 5 volts. The Rx/Tx read 5 volts either way.
So use the 5 volt Vcc line then?
What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?