For computer / microcontroller comms, a serial adaptor is needed. The adaptor shown is the PAS12 adaptor. It converts the +/-13V of an RS232 serial port to +5V and 0V.

Most microcontrollers work at +5V / 0V logic levels or less. This adaptor has been tested with Microchip, Atmel, Motorola, Scenix, Texas Instruments, Philips, 8051 series and Hitachi microcontrollers.

Download the PAS12 datasheet in .pdf format. Feel free to make a serial adaptor yourself, the PCB design is supplied.