آهان خب علتش اینه که due خودش 4 تا سریال سخت افزاری داره و دیگه نیازی به softwareserial نداره عملا .
شما میتونید از بقیه سریال هایش استفاده کنید به جای softwareserial.
The Arduino Due has three additional 3.3V TTL serial ports: Serial1 on pins 19 (RX) and 18 (TX); Serial2 on pins 17 (RX) and 16 (TX), Serial3 on pins 15 (RX) and 14 (TX). Pins 0 and 1 are also connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip, which is connected to the USB debug port. Additionally, there is a native USB-serial port on the SAM3X chip, SerialUSB'.