code: def pack_mbap(transaction_id, protocol_id, length, unit_id):
      
      return struct.pack('>HHHB', transaction_id, protocol_id, length, unit_id)
docs: Create and return response MBAP.
  
      :param transaction_id: Transaction id.
      :param protocol_id: Protocol id.
      :param length: Length of following bytes in ADU.
      :param unit_id: Unit id.
      :return: Byte array of 7 bytes.