RS-232 Protocol

Pinout

The pinout of a male RS-232 DE-9 connector. Image from http://www.ethernut.de/.
The pinout of a male RS-232 DE-9 connector. Image from http://www.ethernut.de/.

Cheap Discrete-Part RS-232 To TTL Converter

A RS-232 to TTL logic-level converter can be made out of a few discrete components. The schematic shown below uses some clever circuitry, including a charge-pump like circuit, to generate the negative voltage required for RS-232 transmission back to the computer.

The schematic of a cheap, discrete-part RS-232 to TTL logic-level converter. Image from Atmel AVR910 (http://www.atmel.com/).
The schematic of a cheap, discrete-part RS-232 to TTL logic-level converter. Image from Atmel AVR910 (http://www.atmel.com/).