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A three-state logic element can drive a digital output high (1), drive it low (0), or disable its output so it presents high impedance (Z). The Z state releases the line for another circuit to use; it is not a third voltage or data value.
What the three states mean
| Output condition | What the circuit does |
|---|---|
| High (1) | Actively drives the output to the logic-high level. |
| Low (0) | Actively drives the output to the logic-low level. |
| High impedance (Z or Hi-Z) | Disables its output drive so the line is effectively released. |
In a typical push-pull output stage, the circuit actively drives both high and low when enabled. In Z, it turns off those drive paths. A real component is not a perfect open circuit: leakage and other electrical limits still apply.
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How a three-state element works
A three-state buffer or gate has a data input and an enable or output-enable control. When enabled, a buffer passes its data input to the output; a gate may instead produce the result of its logic function. When disabled, the output enters Z regardless of the data input.
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Why use a high-impedance output?
Z lets multiple devices connect to a shared bus or line without all of them driving it at once. The selected device drives the line; the others disable their outputs and release it. This is a form of bus sharing: one device talks while the others listen.
The system must coordinate which device is allowed to drive. If two push-pull outputs drive opposite levels simultaneously, they can contend, potentially causing incorrect signals or stressing the circuit.
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What Z does—and does not—mean
- It means the output driver releases the line. It does not actively drive a third voltage.
- It does not create a third data value. The data logic remains binary; the third condition describes the output driver.
- It does not guarantee a defined voltage on the line. Without another driver, a pull-up, a pull-down, or another circuit element setting the level, a released node may float and be undetermined.
Three-state outputs versus open-drain outputs
A three-state push-pull output actively drives both binary levels when enabled and releases the line when disabled. An open-drain or open-collector output instead relies on a pull-up and has different wired-connection behavior. The right comparison depends on the specific circuit and device; check its datasheet before drawing electrical conclusions.
Three-state logic is not ternary logic
The word “three-state” refers to the output’s electrical behavior, not to a logic system with three data values. High and low represent the ordinary binary outputs; Z means the output is not actively driving the line.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Further reading
The Art of Electronics, 3rd edition, discusses three-state logic and bus sharing. Its succinct description is: “It’s just ordinary logic, with a third output state: open circuit.”
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