The plot thickens. I have now noticed that the Uno doesn't handle two-way serial communications correctly, and I think this is related to the autoreset problem.
I have an application which sends a "RESET\n" command to an arduino from a processing application (data logger). The Duemilanove "hears" this command each and every time it is sent, even though it is busy sending a dataset every second back to the host WinXP PC.
But when I loaded the same sketch into the Uno board, the application became broken. I found that the initial "RESET\n" command has to be issued twice or the Uno won't hear it. Then I did some experimenting using the IDE serial monitor.
What I learned is that after a new connection is opened between the PC and the Uno, the first incoming communication is not heard by the Uno. After the first one, all subsequent commands from the PC are heard just fine. It stays this way until a new connection is established between COM13 (in my case) and the Uno.
The other oddity is that the Duemilanove resets every time a new serial connection is established, but the Uno doesn't.
I'm sure these differences are the root cause of the autoreset problem, but I don't know enough about how usbser.sys and the 8U2 communicate with one another to figure out how to fix it.
Anyone?
Jim