This test requires a USB device implementing the USB CDC-ACM protocol configured to loop back TX to RX. For example, this Arduino sketch could be used:
void setup() {
Serial.begin(115200);
Serial.setTimeout(0);
while (!Serial) {
;
}
}
void loop() {
if (Serial.available()) {
char buf[1024]; // Greater than the endpoint packet size.
int count = Serial.readBytes(buf, sizeof buf);
Serial.write(buf, count);
}
}