If, for example, one board firmware uses parity bits and another does not use parity bits and one opens the serial port then depending on the board it would/wouldn't need a parity bit. Before you say that the boards do/don't use parity I am merely using the example of a possible difference in the characters of the data stream. If you do not like the parity example use BigEndian and LittleEndian. Again I do not know of any difference so my question remains, if different boards are selected would the data remain the same. To state that the serial port is simply opened does not answer the underlying question: "Is there any difference in the data?" I am not privy to the internal workings of the IDE and what it may or may not do when the different boards are selected and when a serial port is opened but I am loath the assume that there is no difference.