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SESK: Stuff Everyone Should Know · sesk.info/hame-tri-state-logic

What is tri-state logic?

Outputs of 0, 1 and high impedance.

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  1. Output states of 0, 1 and high impedance ✔
  2. Logic that uses ternary math
  3. A counter with eight states

Why: The third state is a disconnected, high-impedance output. That lets many devices share one bus without fighting over it.

Huh, didn't know that: A pull-up or pull-down resistor sets a known voltage when an input or output is an open circuit (E6C07).

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References

Answer: Output states of 0, 1 and high impedance. The third state is a disconnected, high-impedance output. That lets many devices share one bus without fighting over it.

  1. NCVEC 2024-2028 Amateur Extra Question Pool, E6C03 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    A. Logic devices with 0, 1, and high-impedance output states
    Checked 2026-10-11.
  2. NCVEC 2024-2028 Amateur Extra Question Pool, E6C07 (4th errata, Feb. 4, 2026) www.ncvec.org/downloads/2024-2028%20Extra%20Class%20Question%20Pool%20and%20Syllabus%20Public%20Release%20with%204th%20Errata%20Feb%204%202026.pdf
    B. A resistor connected to the positive or negative supply used to establish a voltage when an input or output is an open circuit
    Checked 2026-10-11.

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