[App_rpt-users] COR

Guys, I'm trying to get COR sorted via the Parallell port. I've figured out I need to ground pin 15 in order to turn RX on and off. The problem is, I have a +5V signal from my radio when there is a carrier present. Can anybody point me toward an easy circuit to be able to convert the +5V to a negative in order to turn the RX on and off? If I set ppinvert, it turns the RX on when I ground the pin - so that seems to be the way to go. I've tried googling, but all sorts of stuff comes up! Thanks, James