I'm trying to use a digitizing probe, and it won't respond using pin 15. Pins 11,12, and 13 were hooked up to limit switches, so 15 was the only one left. The probe would only work when I used it on pin 11, but then I couldn't get the limit switches to work on the remaining pins. For that matter, pin 15 doesn't work at all when 11,12, and 13 are being used.