BASIC QUESTION: Tri state meaning

[ankitgarg0312]

in terms of transistors,

if i say that i have node going from 0 to Z state.
in reality what will be voltage at that terminal ?

my understanding is that by putting node into tristate , i am switching off all paths of current from that node. so node should get remain in its original state forever ( ideally).
i am not sure whether i am right or not ...
please comment

 

[IADanilov]

Assuming that you have no other sources defining node voltage, you are right.

 

[daveguy]

In practice, it might be better to say that you are reducing all current paths to the node, making it high impedance, since even "off" switches will leak some current.

The reason to make that distinction is that as a high impedance node, the smallest amount of leakage charge from anywhere will cause its voltage to vary wildly.

But in its tri-state mode, nobody should care what its voltage is because from a logical point of view, it's not logic true, and it's not logic false; it is in the "third state" of logic - undefined, neither true nor false. That is why it is called "tri-state".

Asking what it's state is in terms of voltage is like asking what is the color of the hair of a bald man - the very question makes an illegal assumption - that its state may always be determined.

However, it's still a good question to wonder about. To measure the voltage, all you need to do is find a voltmeter with an input impedance much higher than the impedance of the tri-stated node.