DigistumpArduino/digistump-avr/libraries/DigisparkCDC/examples/Echo/Echo.ino

#include <DigiCDC.h>
void setup() {                
  // initialize the digital pin as an output.
  SerialUSB.begin(); 
}

// the loop routine runs over and over again forever:
void loop() {
  
  if (SerialUSB.available()) {
    SerialUSB.write(SerialUSB.read());
  }
  
   //SerialUSB.delay(10);
   /*
   if you don't call a SerialUSB function (write, print, read, available, etc) 
   every 10ms or less then you must throw in some SerialUSB.refresh(); 
   for the USB to keep alive - also replace your delays - ie. delay(100); 
   with SerialUSB.delays ie. SerialUSB.delay(100);
   */
}