I'm doing a project and need to write the output vector of an IC. One
particular 74 series IC is 74LS240 which is tri state buffer(http://
rabbit.eng.miami.edu/info/datasheets/74LS240.pdf). However, in the
function table, one of the pin output is high impedence. Does it means
that the output if measured by a voltmeter is low or 0V?
A "high impedance output" is one that is a "weak ouput"; it cannot
impart much "effect" on anything so if you measure it with your
voltmeter then, it is possible that it may not be able to drive the
needle off its stop so it might read 0V but it might not either e.g.
"static electricity" also has a "high impedance".
The opposite of this, a "low impedance output" is a "strong output";
it can impart much "effect" e.g. a car battery.
So without knowing anything about electronics, you could correctly
guess that a watch battery will have a higher impedance output than a
flashlight battery will have a higher impedance output than a car
battery.
The highest sort of output impedance would be... nothing at all.
When a battery is new, its "output impedance" is low. When it gets old
its "output impedance" gets higher.
If you temporarily short-out a dry-cell, its output impedance will
temporarily go high (as gasses build up inside) and then it will
recover its original lower output impedance (as the internal gasses
are absorbed).
The slangy term "output impedance" is more of an "output resistance"
in these cases.
Without a clear understanding of "ouput/input impedance" you won't get
anywhere so you are right to ask - more so than you realise. See
Thevenin's (tay-venins) Theorem.
Robin