three-state logic

In three-state logic, an output device can also be high impedance.

Some digital devices support a form of three-state logic on their outputs only.

Usage of three-state logic is not recommended for on-chip connections but rather for inter-chip connections.

Three-state logic can reduce the number of wires needed to drive a set of LEDs (tristate multiplexing or Charlieplexing).

Chip select (CS) or slave select (SS) is the name of a control line in digital electronics used to select one chip out of several connected to the same computer bus usually utilizing the three-state logic.

A three-state logic device is unlike an open collector device, because it comprises transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.

Pull-up resistors on the motherboard ensure they will remain high (inactive or 'deasserted') if not driven by any device, but the PCI bus does not depend on the resistors to 'change' the signal level; all devices drive the signals high for one cycle before Three-state logic the signals.