A normal logic output is always pushing its wire toward 0 or toward 1. An output in high impedance, written Z, is doing neither. Electrically it's as if it had been unplugged from the wire.
Only tri-state outputs can do this. Memory chips use it on their data outputs:
- not selected (CS inactive) → Z
- selected but writing → Z (the data lines are inputs now)
- selected, reading, with OE active → drives the stored word
Why it exists: so many devices can share one bus. Every device that isn't currently talking sits at Z, leaving the wire to the one that is.
What Z is not:
- It is not 0. A wire whose only drivers are all at Z is floating, and its value is undefined (often pulled to a known level by a resistor).
- It is not a Boolean value you can AND or OR. It describes a connection, not a number.
In timing diagrams Z is usually drawn as a line halfway between 0 and 1.