code: def set_checksum(self):
          
          data_sum = self.cmd1 + self.cmd2
          for i in range(1, 14):
              data_sum += self._userdata['d{:d}'.format(i)]
          chksum = 0xff - (data_sum & 0xff) + 1
          self._userdata['d14'] = chksum
docs: Set byte 14 of the userdata to a checksum value.