HotSync serial cable for Sony Clie T/NR/NX/SL/SJ series
Important note: If you have Sony Clie SL10, SJ20 or SJ30, it is not the same as S series. There are some serial HotSync cables for Clie handhelds that are listed as S & N series. It is NOT what you need. You need a serial cable for T series (if it does not say explicitly SL or SJ series). Same is true for NR70/NX70 models.
RS-232 port requires voltages between +5 and +15 volts for logical 0, and voltages between -5 and -15 volts for logical 1, while TTL voltages are between 0 and +5. Commercially available HotSync serial cables for handheld computers usually have built-in circuit for conversion between TTL and RS-232 voltages. It is very simple, and you can build your own, if you have all proper connectors (http://pc-mobile.net/connectors.htm). This circuit can be made using transistors or IC such as MAX232 or MAX233. MAX233 is better, because it does not require external capacitors. See example on the picture below.
I had experience making interfaces for amateur radios. However, I found cheap HotSync serial cables on eBay.com for as low as $5.99 plus shipping, or you can find them in other online stores (http://www.moonlight-tech.com/shopping/shopexd.asp?id=631, http://pc-mobile.net/clie.htm). While you are shopping for serial cable you may also buy USB HotSync Charging cable, then throw you AC charger away, because you will be able to charge your handheld and synchronize you data with a single USB cable. (This is another story, but this Charging cable takes voltage from USB port to charge the battery of your handheld, while the original Sony USB cable is not connected to +VDC pin, instead you need to use an external AC charger.)
Links to TTL/RS232 interface diagrams (other examples):
http://www.compsys1.com/workbench/On_top_of_the_Bench/Max233_Adapter/max233_adapter.html