Home Automation Domotica Forum Europe, Bwired Forum

Code: Select all

Other way to calculate the checksum:
1. Sum all the payload bytes in a word;
2. Subtract both high and low byte from FFFF;
3. Low byte contains the checksum.

Code: Select all

Other way to calculate the checksum:
1. Sum all the payload bytes in a word;
2. Add high and low byte together into variable total
3. If total is > 255 then substract 255
4. Subtract total from FF;
5. Low byte contains the checksum.

Code: Select all

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
' Check the CRC of the received data
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Private Sub CheckCRC(bData As Byte[]) As Boolean

  Dim bChecksum As Byte
  Dim bDataTmp As Byte[]

  ' first copy the bData - else we loose information
  bDataTmp = bData.Copy()
  ' Remove the preamble
  bDataTmp.Extract(0, 1)
  ' Remove the postamble
  bDataTmp.Pop()

  ' Check if Count > 0
  If bDataTmp.Count > 0 Then

    ' Copy and remove the checksum
    bChecksum = bDataTmp.Pop()

    ' Check if calculation and checksum match
    If CalculateCRC(bDataTmp) = bChecksum Then Return True
  Endif

  Return False

End

Code: Select all

sub CalcChksum {
 my $chkstring = shift;
 # Read last 4 char which should include x43 checksum indicator
 my $chksum = substr($chkstring, -4);
 my $chkout = "";
 my $base = 0xFFFF;

 # Check for checksum character
 if(substr($chksum, 0, 2) ne "43"){
  # If no checksum return C1 no checksum error
  $chkout = "ERR:C1";
  return $chkout;
 }
 else{
  # Otherwise calculate checksum
  if(substr($chkstring, 0, 2) eq "0d"){
   # Strip off carriage return
   $chkstring = substr($chkstring, 2, -2);
  }
  # Get checksum value by removing lead 43
  $chksum = substr($chksum, -2);
  # Convert ASCII check string back to hex and then convert and calculate in decimal
  my @bytes = unpack('C*', pack('H*', $chkstring));
  my $chk = 0;
  # Split into decimal byte values and sum
  foreach my $num (@bytes){
   $chk = $chk + $num;
  }
  # convert decimal check value to hex
  my $outhex =  sprintf("%04x",$chk);
  # Split checksum into hi and lo bytes
  my @pairs = $outhex =~ /../sg;
  my $hibyte = hex "$pairs[0]";
  my $lobyte = hex "$pairs[1]";
  # Perform checksum calculation (FFFF - hibyte - lobyte) and return lo byte of result
  my $chkcalc = substr(sprintf("%04x",$base - $hibyte - $lobyte), -2);
  # If checksum OK set checksum code to OK
  if($chkcalc eq $chksum){
   $chkout = "OK";
  }
  else{
   # If lobyte = FF hibyte appears to be used
   # so check hibyte or return C2 checksum calculation error
   $chkcalc = substr(sprintf("%04x",$base - $hibyte - $lobyte), 0, 2);
   if($chkcalc eq $chksum){
    $chkout = "OK";
   }
   else{
    $chkout = "ERR:C2";
   }
  }
  return  $chkout;
 }
}
# End of sub CalcChksum