well u can make a -5V supply that turns on when the level is 0. that way when the output is high u will see 5V. and when the output is LOW u will see -5V.
I have -5 power supply in my board. But it is not controllable. Is there a switch which is controllable by 0 and output -5V?
I know optical coupler can do this. But it is expensive. If there is any IC off the shelf, it will be the best.
RS-232 (max232) IC with 5.1V zener diode as clamper should able to help. anyway, this will invert the logic - 0 = 5V while 1 = -5V. an invertor should use before rs232 translator if reverse logic requried. hope this help