Yes, I can add that to the next firmware version - to only key in straight mode using the tip connection; then if a mono plug shorts the ring to ground, it would not cause a problem. I can add this as a configuration so it can be enabled/disabled.?
It appears Robin, you are also saying a stereo plug doesn't work? This should not be the case - the connectors are for 3.5mm stereo plugs!
Actually, I am thinking this can be resolved in firmware: If you are running is straight key mode (Menu 4.1 Keyer mode: Straight), then the QCX must just ignore the ring or "Dah" input... Only when you enable other keyer modes does it use the ring input for dah.