You are correct; the dashboard port on the RC signals at +/- 12V. It does this by using a little IC known as a MAX232.
The MAX232 is a voltage level translator specifically designed for the RS232 standard. A quick Google search on the chip number will turn up a datasheet.
Use is quite straightforward--after connecting a few required capacitors, connect the RS232 RX pin to the RC dashboard port, and the LVTTL RX pin to your handheld's RX pin.
Best of all, you can request a free sample of the chip in a DIP package from Maxim's website at
www.maxim-ic.com!
Hope this helps...
Eldarion
EDIT: As mentioned below, the MAX3233E is definitely a better choice for this application. Having cut my teeth on the venerable MAX232, I tend to use it for everything...
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