Level conversion b/w RS232 & XBee module using 74LS04(Hex invertet) not working!

Hi All,

I have encountered a problem at the very initial stage using XBee modules.. i want to upgrade its firmware to ZNet 2.5 OEM without using a kit.. For this purpose, i've installed X-CTU & downloaded all latest firmwares..I used hex inverter(74LS04) for level conversions b/w RS-232 & CMOS based modules... but the problem is that its not communicating with modules via serial port.. Can any one plzz help me out... what procedure i actually need to follow for a successful voltage level conversions!!

Thnxx!