Troubleshooting serial port on Windows

Hi all,

I need suggestions on how to troubleshoot serial port issues on Windows.

I have to port an arduino project from Mac to Windows. Works on the Mac, and data comes in from the Arduino through the serial perfectly fine. On Windows it does not work, Max seems to open the port, but no data is coming out of the serial object.

The port d: COM4 is correct, as shown in the Max console.
If I have the serial monitor on the Arduino running and open Max, I get the error message in Max that the port is already open. If I open the Max patch and then open the Arduino serial port I get the same error message from the Arduino. I conclude that the port is open. That said, no matter how often I bang the serial port, there is no data coming out of the serial object.

I loaded the pieces of software in different order: Max only, or Arduino then Max, or Max then Arduino, none of them make a difference. I played around with the settings in the serial objects' help file, no change. I restarted everything, and in a different order. That serial thingy just sits there smirking at me doing nothing. ;-)

Any ideas where to look? It's a PC with Windows 10, latest Max, latest Arduino IDE.

Hans