def generate_checksum_bytes(payload)
      sum = payload.reduce :+
      sum_hex = ('%04x' % sum).upcase

      string_to_ascii_bytes sum_hex
    end
def generate_checksum_bytes(payload)
      sum = payload.reduce :+
      sum_hex = ('%04x' % sum).upcase

      string_to_ascii_bytes sum_hex
    end
Returns a checksum string (4 characters) appropriate for sending to the serial port

 ie: '12AF' (note capitalization)