Native Mode Checksum

The checksum that is used by Native Mode file transfer commands is a standard 16-bit Cyclical Redundancy Check (CRC). The algorithm for calculating the CRC follows. In this example the variable CRC is unsigned 16 bits and the input byte is 8 bits.

  1. Set CRC to 0.
  2. Get next byte from data stream.

  3. Test the most significant bit of CRC and the input byte and do the following:

         If they are equal shift CRC left 1.

         If one bit is 0 and the other 1 then shift CRC left 1 and XOR with 4129.

  4. Shift the input byte left 1.
  5. If you have not shifted the input byte 8 times go to step 3.
  6. If you have remaining bytes in the data stream go to step 2.

You can find a theoretical explanation of this algorithm in Numerical Recipes in C:The Art of Scientific Computing.1
A C function that calculates the CRC one bit at a time follows:

unsigned short CalculateCRC(char* buffer, int size)
{
  unsigned short cword = 0;
  unsigned short ch;
  int i,j;
  for (i = 0; i < size; i++) {
    ch = buffer[i] << 8;
    for (j = 0; j < 8; j++) {
      if ((ch & 0x8000) ^ (cword & 0x8000)) {
        cword = (cword << 1) ^ 4129;
      }
      else {
        cword <<= 1;
      }
      ch <<= 1;
    }
  }
  return cword;
}

The vision system calculates the checksum of a transferred file from the ASCII character values of the hexadecimal encoded file contents. For example:

File Contents 0xAB 0xCD 0xEF 0x01
ASCII Representation ABCDEF01
Byte Values to Check 0x41 0x42 0x43 0x44 0x45 0x46 0x30 0x31

This encoding for file transfers over Native Mode commands adds a sequence of a carriage return and a line feed character after every 80 visible characters.