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I came across a simple hex inverter IC with the truth table having "z" as an output state for a low (L) input. So what does Z stand for here? Does it mean a toggle or partial state? The other variant of this IC which does not use open drain outputs has the usual Function table of an inverter.

Function table(usual):

Input|output
L | H
H | L

Function table(with open drain):

Input|output
L | Z
H | L

The device description is as :

"The device contains six independent inverters. These devices perform the Boolean function Y = A. The open-drain outputs require pull up resistors to perform correctly, and can be connected to other open-drain outputs to implement active-low wired-OR or active-high wired-AND functions"

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up vote 13 down vote accepted

Interestingly, none of the other answers tell you the main purpose of this inverter.

They are all correct : this is a high impedance state. And as you quote in the question, this device needs a pull-up resistor to work.

So why use a gate that needs an extra component?

Because it's a way of sharing signals, of connecting several signals (possibly in different locations) together, to share a common wire. This has various purposes, including communication in both directions on a single wire, as in the I2C bus (which has a second wire for the clock). Or allowing an unknown number of connections to the signal : you can always plug another one in, allowing hotplug connections.

Consider what happens if you connect 2 conventional logic signals together : if one drives "H" and the other drives "L", they fight each other, the actual voltage can be indeterminate, the stronger driver usually wins, and it's possible to burn the other one out... not good.

But connect 2 of these together (with the required pullup resistor) - if either or both is '0', the value is '0'. If all outputs are 'Z', the value is 'H'. That's it.

It's commonly called a "wired-OR" or sometimes a "wired-AND" configuration - though if all the drivers are inverters as in your case, it's actually a wired-NOR structure. Draw out the truth table to confirm this...

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It represents a high impedance (Z) output state. The output will (ideally) neither sink nor source current, nor can it act as a voltage source.

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Z = high impedance state.

An "open collector output" is just that - the output is a transistor collector pin. NPN type, more specifically, with the emitter tied to ground. So that collector only has two states:

  • Signal feeding it's base = off, transistor in cutoff, collector = high impedance.
  • Signal feeding it's base = on, transistor saturated, collector = ~Vss or Low.

So this is why the datasheet says "H" or "Z" for the other state - in that state, it will be high-impedance (a very high resistance, like 100MΩ) since the transistor is off. "H" because typically, the whole point of open-drain outputs is to "pull up" this state to some logic high voltage. Often, it is a different voltage than Vdd.

schematic

simulate this circuit – Schematic created using CircuitLab

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Note that putting 10V to an output that isn't designed for it, even an OC output, can have a negative effect on the IC. – Ignacio Vazquez-Abrams Jan 22 at 12:57
    
True, this was just for demonstration purposes. Always check with the device datasheet to make sure it won't pop and release the magic smoke. – rdtsc Jan 22 at 13:00
    
Note also there are some specific members of the (5V) 74 series logic family, with reinforced output stages that can tolerate up to 30V. They were used to drive fluorescent displays among other purposes. As Ignacio points out; use these if you want to put 10V on the outputs... – Brian Drummond Jan 22 at 13:01
    
And also i wonder the Base Resister 100 Ohms(R1,R2,R3,R4) it usually will be in Kohms. The Base current << collector currnet. If people may guess for shake of understanding mean no need of scratching the head – Raj Jan 22 at 13:14
    
True, but again, this is a demonstration, not any actual device. – rdtsc Jan 22 at 16:31

In Brief

Z mean "High Impedance State "

In Deep,

The inverter which you are seeing is tri-state inverter.
High - Output Goes to 5V or depends on Vcc Low -0v High Impedance State- Consider as open circuit.

For Better understanding consider this image(This is not inverter for understanding i added tri-state buffer which is also in same concept) enter image description here

When Enable=0 it will be in High impedance state. In this state whatever may be the input, the output won't reflect.Because it becomes opened circuit, isolated from the supply.

Whenever Enable=1 (some times it will be in active low) that time it will act as normal digital logic

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Z stands for High impedance.This means that,it's a open circuit.

    In digital electronics,there are three forms of output and inputs
    1.HIGH state
    2.LOW state
    3>HIGH IMPEDANCE state

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