I believe the last post describes the reason behind the shift in serial ports that I have been seeing while programming the third of three MKR Zeros I have. The first two presented on real problems, but the newest one has. The board seems to accept the code, but as soon as upload is complete, the comm port changes and the ability to use the serial port for debugging is lost. Re-selecting the original comm port doesn't establish communication from the board. Curiously, a few example programs do work with serial.print despite a warning from the IDE that no board can be found on that port. Also, the buttons are grayed out on the serial monitor window as if it isn't recognized, but the information from the RTC example still prints. Hope someone can tell me how to get around the problem. BTW: I am using the latest core (1.6.19) for the board and the latest IDE version 1.8.8.