Re: vhdl
Think about what you're asking. As FvM mentioned, 'Z' really only makes sense for an output. When you drive a 'Z' onto an output you are tri-stating that output.
What this means is that the output driver is shut off. It's no longer driving a 1 or 0, it's doing neither. The driver isn't sinking or sourcing any current. In an ideal case without a pull-up or pull-down resistor on the net and absolutely no leakage current, the last state driven onto that net will remain there indefinitely.
In the real world, tri-state cannot be sensed. A net is either a 1 or 0, or has some value between Vil an Vih which is really an unknown state. For input values within that range most receivers will either detect a 1 or a 0 but those values are meaningless.
Radix