/*
 *  AX.25 protocol frame structure:
 *  -------------------------------
 *
 *  Flag Field: 8 bits (1 byte)
 *  Header Field: 128 bits (16 bytes)
 *      {
 *    Destination Address: 56 bits (7 bytes)
 *    Source      Address: 56 bits (7 bytes)
 *    Control Bits       : 8  bits (1 byte )
 *    Protocol Identifier: 8  bits (1 byte )
 *  }
 *  Data Field: 32-2048 bits (4-256 bytes)
 *  Frame Checksum Field: 16 bits (2 bytes)
 *  Flag Field: 8 bits (1 byte)
 *
 */

unsigned char AX25_Flag;
unsigned char AX25_Header[16]  = {0};
unsigned char AX25_Data[256]   = {0};
unsigned char AX25_CheckSum[2] = {0};

int inByte = 0;

void setup()
{
    // Serial1 : TX pin 18, RX pin 19
    // Serial2 : TX pin 16, RX pin 17
    // Serial1 will receive data from a device and resend it on Serial2 using AX25 protocol format

    Serial1.begin(9600);
    Serial2.begin(9600);
}

void loop()
{
    // Wait until data is received on Serial1 port
    while ( !(Serial1.available()) );

    // Read received data
    inByte = Serial1.read();

    // Send the data using AX.25 protocol format
    AX25_Data[0] = (unsigned char) inByte & 0xFF;

    Serial2.write(AX25_Flag);
    Serial2.write(AX25_Header, 16);
    Serial2.write(AX25_Data, 256);
    Serial2.write(AX25_CheckSum, 2);
    Serial2.write(AX25_Flag);

    delay(500);
}