Hmmm... The docs mention DTR enable That would imply that you must enable DTR (pull pin 4 high) during communication like with the MSCOMM DTREnable option. I'm not sure that the Pronto serial extender has a similar option via the Serial class. Perhaps this would need to be handled in hardware by making an adapter that always pulls this line high. Another thing to check is if the DTR pin is asserted high by default by the serial extender. If so then there is probably a software problem. If it is low then that would be the first thing I would work on.