I am a newby, and I have an Uno, running Arduino 0021 on Ubuntu 10.10.It worked fine running the example “Blink” and some simple variations (I could change the blink times, so I know code was uploading), but after I uploaded the example “DigitalReadSerial” the TX LED always on.    Pushing the reset button causes the “L” LED to briefly flash, but the TX light stays on any time power is applied.  The RX LED does not come on.When I try to open the serial connection or upload I get a message “Serial port ‘/dev/ttyACM0’ not found.  Did you select the right one from Tools > Serial Port menu?”  When I look in the Tools menu, the Serial Port selection is greyed out.I rebooted my PC, but that didn’t fix things.  I also tried a different USB port, but no difference.Any ideas on what is wrong, or how to fix it?Here are a few port diagnostics (I have no idea what they mean):

Source: TX stuck ON, serial port greyed out