Sparkfunkit
David @ uCHobby writes in –

Here’s how to build a simple TTL to RS232 adaptor for your microcontroller projects. You should be able to scrounge all the parts you need or buy them at your local Radio Shack. In this article I explain how the adaptor works and provide many links to more information you need to know as a microcontroller hobbyist. This article should be very helpful to those that receive the free Arduino kit offered by uC hobby as part of the Arduino Microcontroller kit giveaway.

uC Hobby » Blog Archive » TTL to RS232 adaptor Explained – Link.