Calculate checksum of NMEA-protocol

schimanski

Well-Known Member
Licensed User
Longtime User
Hello together!

I have created a sub, which add a checksum to a NMEA-protocol. My question is: is there an easier way to calculate the checksum? I think, that my sub sometimes works and sometimes it doesn`t.


NMEA is the NMEA-protocol as string without the checksum,

B4X: 
Sub Globals
  Dim buffer(0) as Byte
End Sub

Sub Checksum_NMEA
  buffer()=bit_NMEA.stringToBytes(NMEA,1,StrLength(NMEA)-2)
  laenge=ArrayLen(buffer())
  summe=buffer(0)
  for j=1 to laenge-1
    summe=bit_NMEA.XOR(summe,buffer(j))
  next
  checksum=bit_NMEA.DecToHex(summe)
  NMEA=NMEA & checksum & crlf
End Sub

Thanks for an answer

schimanski
 

agraham

Expert
Licensed User
Longtime User
schimanski said:
I think, that my sub sometimes works and sometimes it doesn`t.
Click to expand...
Looks and tests fine for me, admittedly on a limited set of data. However I think that the checksum should always be two characters and your Sub could fail because bit_NMEA.DecToHex doesn't return two characters for a number less than 16 so you should check for this and correct it.

B4X: 
  ...
  checksum=bit_NMEA.DecToHex(summe)
  If summe < 16 Then
    checksum = "0" & checksum 
  End If
  ...
 

schimanski

Well-Known Member
Licensed User
Longtime User
Thanks, agraham!

I will correct my code. This could be the answer, why my navigationsoftware tomtom sometimes accept the protocoll and sometimes not.
 
W

wolfgang

Member
Licensed User
Longtime User
NMEA chksum

Hi,
I did this a couple of months before and shared it here in the forum:
B4X: 
Sub Globals
   'dll: bitwise.dll, object: bit
   nmea = "$GPGGA,102334.994,4841.7903,N,01004.0063,E,0,00,50.0,572.3,M,,,,0000*3F" 'example
   Dim checksum
   n = 1
End Sub

Sub App_Start
   Form1.Show
   bit.New1
   sign = StrAt(nmea,1)
   Do Until sign = "*"
      sign = Int(Asc(sign))
      checksum = bit.XOR(checksum,sign)
      n = n + 1
      sign = StrAt(nmea,n)
   Loop
   Msgbox("ASCII:  " & checksum & CRLF & "HEX:    " & StrToUpper(bit.DecToHex(checksum)))
End Sub
 
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