def read_holding_registers(slave_id, starting_address, quantity):
function = ReadHoldingRegisters()<EOL>function.starting_address = starting_address<EOL>function.quantity = quantity<EOL>return _create_request_adu(slave_id, function.request_pdu)<EOL>
Return ADU for Modbus function code 03: Read Holding Registers.

    :param slave_id: Number of slave.
    :return: Byte array with ADU.