I got a breadboard with some logic on it. Including it's own power suppply. It is connected to my Arduino Mega 2560 to fill a shiftregister with some bits. Whenevery my arduino is in a undefind state i want the shiftregisters to be dissabled to prevent the hardware from hazards. Undefind means when it is turned off. Since the logic has it's own powersupply it could damage some of the hardware which i toggle with the registers. (toggling transistors with a lot of leds on it)
So i added a pull up to the "blank pin" of the shiftregister(2k 10k 18k). But it does not work as expected. In this case it dows nothing(The test leds still are in some kind of undefind state when i turn off the power of the arduino)! If i delete the gnd line between the arduino and the breadboard it works.
So here is some small shematic (Not compplete but the important things):
What am i doing wrong?
Edit. Sorry i am not that good in drawing this schematics...