digitalWrite()とSerial1.write()の切り替えについて

void setup() {
    const byte txData[6] = { 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF };

    //Serial1.write (1回目)
    Serial1.begin(9600);
    Serial1.write(txData, 6);
    Serial1.end();
    delay(1000);

    //digitalWrite
    pinMode(18, OUTPUT);
    digitalWrite(18, LOW);
    delay(50);
    digitalWrite(18, HIGH);
    delay(100);

    //Serial1.write (2回目以降)
    for(byte i = 0; i < 5; i++)
    {
        Serial1.begin(9600);
        Serial1.write(txData, 3);
        Serial1.flush();
        Serial1.end();
        delay(1000);
    }
}

void loop() {
    
}

AA BB CC DD EE FF
AA BB CC
AA BB CC
AA BB CC
AA BB CC
AA BB CC

AA BB CC DD EE FF
00