An active-low signal performs its named action when its level is zero. A tri-state output can drive low, drive high, or enter high impedance, often written Z. High impedance lets another enabled driver control a shared wire; it is not a third Boolean value.
Worked example
Suppose RESET_n = 0 asserts reset, while RESET_n = 1 releases it. On a shared bus, device A can be enabled while device B is high impedance. Before B drives, disable A so the devices do not drive conflicting levels.
What to watch for
A floating high-impedance wire may need a pull-up or pull-down. Two ordinary outputs driving opposite values can create contention rather than a meaningful logic result.
Try it yourself
What does the suffix _n tell you about ENABLE_n?
Answer: The enable action is active when the signal is low. A value of one deasserts that enable.
How to study this topic
Begin with settled Boolean values, then consider the physical signal and the device's electrical limits. A logic symbol explains a function; its data sheet explains which voltages, currents and delays are permitted.
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Digital Electronics