Hi Doug,
I'd also advise you to try the following:
In the device manager, locate the corresponding "mbed Serial Port" device. If you double-click on it, you can go to the "Port Settings" tab, then "Advanced". Here you have an option to change the COM port number. Try to map the mbed to another (free) COM port, and see if it does the trick (as COM ports might sometimes get in conflict between devices).
If that fails, you can try uninstalling the mbed from this device manager before trying to reinstalling it with the serial driver.
Also do you have the option of testing that mbed on another computer?
Have you installed new hardware on your PC or upgraded some drivers recently?
Hope this helps,
Donatien
I have three LPC1768 modules on my bench. Something has happened to one module in that it no longer shows a COM port when Tera-Term is loaded. (TeraTerm says "Cannot open COM3"). When this first happened, TeraTerm suddenly froze while receiving data. After closing and reopening TeraTerm and resetting the mbed device, the "Cannot open COM3" message first appeared. The rest of the module works OK. I can upload new firmware to the mbed folder and everything else, but I now have no debug output with this module. My other two modules work OK so I don't suspect the PC or mbed USB drivers are at fault.
I reformatted the mbed module according to the thread "Dead mbed?" at http://mbed.org/forum/mbed/topic/399/?page=1#comment-1996. The module reformatted without any problems, accepted and ran new firmware, but COM3 could still not be opened. I've powered down both the PC and the module but that did not help. It's like the hardware handling the USB on the LPC1768 has been damaged. Or, is there an unseen bootloader or monitor in the module which may have been corrupted and in need of reloading, or do you think I have a damaged part?
Doug