Ecco il codice, estratto dal file CDC.cpp del core:
// This operator is a convenient way for a sketch to check whether the
// port has actually been configured and opened by the host (as opposed
// to just being connected to the host). It can be used, for example, in
// setup() before printing to ensure that an application on the host is
// actually ready to receive and display the data.
// We add a short delay before returning to fix a bug observed by Federico
// where the port is configured (lineState != 0) but not quite opened.
Serial_::operator bool() {
bool result = false;
if (_usbLineInfo.lineState > 0)
result = true;
delay(10);
return result;
}