sub checksum ($) {
   my $string = shift;
   my @chars = split //, $string;
   my $sum;
   $sum += $_ & 0x7F for (@chars[0..@chars-5]); # add ASCII values up
   $sum += hex(join('', @chars[@chars-4..@chars-1]));
      # add the last four characters interpreted as hex number
   return (($sum & 0xFFFF == 0) ? 0 : 1);   
}