Serial.available()

int incomingByte = 0;   // for incoming serial data
 
void setup() {
        Serial1.begin(9600);     // opens serial port, sets data rate to 9600 bps
}
 
void loop() {
 
        // send data only when you receive data:
        if (Serial1.available() > 0) {
                // read the incoming byte:
                incomingByte = Serial1.read();
 
                // say what you got:
                Serial1.print("I received: ");
                Serial1.println(incomingByte, DEC);
        }
}

void setup() {
  Serial.begin(9600);
  Serial1.begin(9600);
 
}
 
void loop() {
  // read from port 0, send to port 1:
  if (Serial.available()) {
    int inByte = Serial.read();
    Serial1.print(inByte, BYTE);
 
  }
  // read from port 1, send to port 0:
  if (Serial1.available()) {
    int inByte = Serial1.read();
    Serial.print(inByte, BYTE);
  }
}