Hallo Hans,

there is a big confuse about RS-232 compatibility arround. Pure RS-232 standard demands for logical high level ('mark') at output a signal voltage of -5V...-15V. At input -3V...-15V is accepted. For logical low level ('space') at output a signal voltage of +5V...+15V is demanded. At input +3V...+15V is accepted.
BUT: If you have a look at traditional RS-232 transcievers like MAX232 you will find in datasheet that their input threshold low voltage is 1.2V minimum and their input threshold high voltage is 1.7V. That's far away from pure RS-232 standard!
On the other hand output voltage swing of those chips is +-5V minimum, which again fullfills RS-232 standard.
Beside the big advantage of big dead zone between 'mark' and 'space' (that's unfortunately not really used by most transcievers) there is another big advantage of the use of RS-232 transcievers, their drastically decreased slew rate of output signal. This gives you the chance of realizing serial data transmission with highly reduced EMI.

Bye,
Kai