I'm working on a prototype which is using a total of 64 MUX controlled by FPGAs. MUX are powered with +/-12V. I started with assembling one MUX and tested it with success, I could easily control all switches. I then added 3 more MUX. What I observed is that -12V is forced onto some of my MUX inputs when power is on (when power is off, the is no electrical connection between input gates and supply). For the moment I'm using 6 out of 8 inputs, and 2 among these exhibit this behavior. I also put together a complete circuit with 64 MUX, for which all inputs exhibit this behavior. Any idea as to what might be my problem / troubleshooting strategies?