Seatalk1 to NMEA0183 with Arduino

[jontys]

In response to requests on the How Do I Thread about Seatalk1 to NMEA0183. Here's a few details of what I've done with an Arduino Mega.
My implementation is to be an interface between an ONWA plotter using NMEA V3 and a B&G autohelm using NMEA V1.5. 
I will write it up more thoroughly when I've finished it.
The Onwa has an environmental screen which displays wind vector, temperature, humidity etc., So, I am providing that data too. My wind instrument is a Raymarine I60 outputting Seatalk1.
Using snippets of code from the Seatalk display code on http://berreizeta.blogspot.com/2017/02/s...nitor.html It's a relatively simple task to extract the wind speed and angle. then build NMEA sentences.
I'm currently using a rather rough and ready seatalk interface of a 4k7 resistor and a couple of diodes to clamp the input voltage to 0.6v of the arduino supply rails. The I60 output is 5 to 6 volts anyway, but, I intend improving the spike protection before using this in anger. The input feeds directly into the receive pin of Serial port1.
I'm using Nick Gammon's 9 bit serial library. NB: you do need to use no newer than V1.6.9 of the Arduino IDE for this library. And, if you already have a newer version installed, it can be a hunt to find the right location to place Nicks library!
Output to the ONWA is via Serial port2 and a MAX485 adaptor.
Output to Openplotter is via the USB Serial Port.
Code Below:

/*Seatalk 1 Input to NMEA MWV sentence output
 * Requires 9 bit serial support, which only compiles correctly on Arduino IDE V1.6.9 or earlier
 */
char SeaTalkData[256] , index;
int z, inbyte , windangle;
float windspeed;
String NMEA = "       ";
byte CRC=0;

void setup ()
{
 Serial.begin (4800);  // debugging prints
 Serial1.begin (4800, SERIAL_8N1,true);  // 9 bit mode. Serial port1 input connected to seatalk data out from Raymarine I60 via 4k7 resistor.
 Serial2.begin(4800); //Output to NMEA Plotter via MAX485 convertor. TX/RX held High
 Serial.println ("--- starting ---");
}  // end of setup

void loop()
{
 if (Serial1.available ()) {
    inbyte = (Serial1.read());

    if (inbyte & 0x100){
      datagram();
      index = 0 ;
    }
       if (index == 0) {
       //Serial.println() ;
    }  // end 0x100
    
    SeaTalkData[index++] = inbyte ;

 }  // end serial
 
 }  // end loop
void datagram(){
  //-----------------------------------------------------------------
// Apparent Wind Angle (AWA): 10 01 XX YY
   if (SeaTalkData[0] == 0x10){
     windangle = (makeWord(SeaTalkData[2] , SeaTalkData[3]))/2;
     if (windangle > 360){ windangle = 360;}
    }  // end 0x10
//-----------------------------------------------------------------
  // Apparent Wind Speed (AWS): 11 01 XX 0Y 
  //   (XX & 0x7F) + YY/10 = speed
  //   XX&0x80=0           = knots (default)
  //   XX&0x80=0x80        = meters/second
  if (SeaTalkData[0] == 0x11){ 
  windspeed = ((SeaTalkData[2] & 0x7F) + (float(SeaTalkData[3]) / 0x0A));
    if (windspeed > 99){ windspeed = 99;}
    if (windspeed < 0){ windspeed = 0;}
//-------------------------------------------------------------------
   //Generate wind sentence
  NMEA="XXMWV,";  //Wind angle & speed
  NMEA=NMEA+ String(windangle); //wind angle 0 to 359
  NMEA=NMEA+",R,"; //Relative or True
  NMEA=NMEA+String(windspeed); // Wind Speed
  NMEA=NMEA+",N,A";  //Units Km/Metre/kNots and Valid indicator  
  CalcCRC();
  Serial2.println(NMEA);
  Serial.println(NMEA);

  }  // end 0x11 

}
void CalcCRC(){
    //Using the NMEA string without Dollar sign and asterix
    //Create a CRC by doing bitwise XOR with each character in the string.
    CRC=0;
    for(z=0;z<NMEA.length();z++){
      CRC=CRC ^ NMEA[z];
    }
  NMEA="$"+NMEA;  //Place Dollar at the start of string
  NMEA=NMEA+"*";  //Place asterix at the end of string
  if(CRC<16)NMEA=NMEA+"0"; //If CRC is less than 16 then the Hex output would be 1 charcter - Pad the 1st character with zero
  NMEA=NMEA+String(CRC,HEX); //Append the CRC
}