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High impedance (also known as hi-Z, tri-stated, or floating) is the united state of an output terminator which is not currently driven by the circuit. In digital circuits, it means that the signal is neither driven to a logical high nor low level - hence "tri-stated". Such a signal can be seen as an open circuit (or "floating" wire) because connecting it to a low impedance circuit will not affect that circuit; it will instead itself be pulled to the same voltage as the actively driven output. The combined input/output pins found on many ICs are actually tri-state capable outputs which have been internally connected to inputs. This is the basis for bus-systems in computers, among many other uses.
In digital circuits, a wire at high impedance may sometimes have a voltage that is around, or even lower than, the threshold for a digital 0. This can cause people to mistakenly read a hi-Z wire as a digital 0. To verify if a wire is at hi-Z, a large value pull-up resistor can be used to try to pull the wire to high and low voltage levels. A non-hi-Z signal should not be able to be pulled in such a way.
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