Arduino modbus RTU frame sender (for test purpose)

modbus_send.ino

/*
  Arduino modbus RTU frame sender (for test purpose)
 */

// some modbus frames
const uint8_t f1[] = {0x0a, 0x03, 0x00, 0x00, 0x00, 0x01};
const uint8_t f2[] = {0x0b, 0x03, 0x00, 0x00, 0x00, 0x10};

// some functions
uint16_t crc16(const uint8_t *buf, uint8_t len)
{
  uint16_t crc = 0xFFFF;
  for (int pos = 0; pos < len; pos++)
  {
    crc ^= (uint16_t)buf[pos];
    for (int i = 8; i != 0; i--) 
    {
      if ((crc & 0x0001) != 0) 
      {
        crc >>= 1;
        crc ^= 0xA001;
      }
      else
        crc >>= 1;
      }
  }
  return crc;
}

uint16_t send_frame(const uint8_t *buf, uint8_t len)
{ 
  // compute CRC
  int16_t crc = crc16(buf, len);
  // send frame body and CRC
  Serial.write(buf, len);
  Serial.write(crc & 0xff);
  Serial.write(crc>>8 & 0xff);
}

void setup() 
{
  Serial.begin(9600);
}

void loop() 
{
  send_frame(f1, sizeof(f1));
  delay(500);
  send_frame(f2, sizeof(f2));
  delay(500);
}