Standard RS232 signals can range from -15V to +15V, so you must never directly connect any RS232 device to the GPIO serial pins. You must use an RS232 to TTL voltage-level converter (such as a MAX232 chip) or a device that uses TTL-level signals (such as another microcontroller or a TTL serial console cable):
A USB-to-TTL serial console cable (voltage level is 3V)
Raspberry Pi has TTL-level serial pins on the GPIO header that allow the connection of a TTL serial USB cable. The wires will connect to the Raspberry Pi GPIO pins and the USB will plug in to your computer and be detected like a standard RS232-to-USB ...
