code: def read_holding_registers(slave_id, starting_address, quantity):
      
      function = ReadHoldingRegisters()
      function.starting_address = starting_address
      function.quantity = quantity
  
      return _create_request_adu(slave_id, function.request_pdu)
docs: Return ADU for Modbus function code 03: Read Holding Registers.
  
      :param slave_id: Number of slave.
      :return: Byte array with ADU.