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High impedance
In electronics, high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current, high voltage, whereas low impedance circuits are the opposite.
==Digital electronics==
(詳細はdigital circuits, a high impedance (also known as hi-Z, tri-stated, or floating) output is not being driven to any defined logic level by the output circuit. The signal is neither driven to a logical high nor low level; this third condition leads to the description "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 (resulting in three-state logic or four-valued logic). This is the basis for bus-systems in computers, among many other uses.
The high-impedance state of a given node in a circuit cannot be verified by a voltage measurement alone. A pull-up resistor can be used to try to pull the wire to high and low voltage levels. If the node is not in a high-impedance state, extra current from the resistor will not significantly affect its voltage level.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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