What is the disadvantage of leaving the inputs of a logic gate unconnected?
The unconected gate becomes a 1 and there is a possibility the gate will not work as desired. This is the case for a NOR gate Unconnected inputs can pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs should be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit! Unconnected inputs are not a logic low; they are not predictable and usually (but not always) float to the high input state. Inputs should always be connected to a gate output, a Logic High (+5V), or a Logic Low (0V or Ground).
