Checksum help please~

Zap-o

I have made a sensor that uses a propeller for measuring the earths magnetic field. My intention was to send these data to a computer that is running a program to chart the data. My issue is the checksum. I have kind of an idea how it works when receiving data but I am really stuck with sending.

1.) from my best guess the program uses NMEA sentences but what kind of Checksum I am unclear, is there not multiple types of Checksum, my wiki research shows all sorts - but again I could be wrong.

Here is a sample of the string I am sending and I hope is in a NMEA style.

  $GPHDT,120.00,T*00<CR><LF>

And here is the crude method currently packing it all together into a string, but as you can see I have 00 as the Checksum because I cant calculate a value - I have no idea where to begin.

Serial.str(String("$GPHDT,")) 
  Serial.str(Math.FloatToFormat(MAgnetic_North,7,3))
  Serial.Char(",")
  Serial.Char("T")
  Serial.Char("*") 
  Serial.Char("0")
  Serial.Char("0") 
  Serial.char(13)
  Serial.char(10)

So I ask my fearful coders to offer some ideas and most importantly something I can validate and check or use to make this function work.

Thanks in advance

Tracy Allen

Try XOR together all the bytes after the $ and before the *.
Test against this calculator:
http://nmeachecksum.eqth.net/

$GPHDT,120.00,T*06<CR><LF>

Mark_T

This is a tool to reverse engineer checksums: reveng.sourceforge.net/readme.htm

pmrobert

It's a relatively simple algorithm - see https://stackoverflow.com/questions/32076761/nmea-checksum-calculation-calculation for details and a short discussion.

Mike R...

ChrisGadd

The NMEA checksum is all message bytes between the $ and * xor'd together.

I'm attaching a simple generator I made for testing a decoder object, uses canned messages but generates the checksum.
The relevant bit is:

PRI Send(stringPtr) | cksum
  cksum := 0
  tx("$")
  repeat strsize(stringPtr)
    tx(byte[stringPtr])
    cksum ^= byte[stringPtr++]
  tx("*")
  hex(cksum,2)
  tx($0D)
  tx($0A)

Zap-o

Thank you all. I believe I am on my way to solving this issue. Again thanks