Square wave -5 to +5 volt at 500 Hz
The simplest way I can see is to use an 1 op amp as a comparator. (probably cheaper than a dedicated RS232 chip and more available especially at local shops)

Have a look at Comparitor Operation. 

When V+ > V- the output will go to the positive rail (+5V)
When V+ &lt; V- the output will go to the negative rail (-5V)

You can set a threshold at V-, like at 2.0V, so that the output will only change when V+ is greater than 2V or less than 2V.