code: def _create_mbap_header(slave_id, pdu):
      
      # 65535 = (2**16)-1 aka maximum number that fits in 2 bytes.
      transaction_id = randint(0, 65535)
      length = len(pdu) + 1
  
      return struct.pack('>HHHB', transaction_id, 0, length, slave_id)
docs: Return byte array with MBAP header for PDU.
  
      :param slave_id: Number of slave.
      :param pdu: Byte array with PDU.
      :return: Byte array of 7 bytes with MBAP header.