Easy, just send a 0x00 to 0xFF out the serial port. It will give you some were from +/- 5V to +/- 15V, most likely +/- 9V. It will be slighly off becuase of the start bit - set it up for no parity and only one stop, but it will be a good approximation. The RTS idea is also pretty good, but liekly to be too slow to drive anything but the simplest loads.
It also would not be a big deal to simply design a little RS-232 powered dongel with a little "pic" controller that could generate the signal. You may have to have some eletronics anyway depending on what you are trying to drive. Are you trying to drive a FET?
Steve Owens
Finish Line Product Development Services
a 2 chip solution could be had by using a serial D/A converter driving a PWM device. Check Maxim site for possibilities. I'm sure you can get a 12bit version for good resolution.