What does "3 state output mean"? I know there is HIGH and there is LOW, what is #3?

 

#3 is open circuit so you can have a bunch of outputs on a single wire and activate only one while the rest are disconnected by their 3-state.

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Uncle $crooge

 

So it is serial? I am confused. What does "Open circuit mean"? Sorry if these questions are dumb, but you must give me credit, i am trying to learn.

 

Open circuit means it is completely disconnected. You cannot connect two ordinary outputs together because they fight about whether their output is high or is low. But if you open circuit one of the outputs then there isn't the problem.

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Uncle $crooge

 

so it is muliplexing?

 

not to quibble but the third state of a tri-state pin is usually described as high impedence (often called high-Z). On microcontrollers, it is done by changing the pin to an input. many logic chips have a high z contol pin. It doesn't truely disconnect the pin, though it draws an insignificant amount of current so for design purposes, you can ignore it. On a pic, you set the coresponding bit in the TRIS registor - TRIS is short for tri-state.

 

Ok, i have heard of high impendence. What does that mean though? I looked up the definetion, but led me to more questions? Can you explain the tri-state pins to me please? Also, could you give examples? Thanks for all of your help so far!

 

HIgh impedence means high resistance- disconnected. Impedence is "complex resistance". It's resistance with a real and imaginary number parts. Resistance usually means impedence with 0 for the imaginary part.

So you the states you have are a 1, 0, and disconnected (or floating).

Pins sometimes have a high impedence state so you can use pull-resistors on the line. You output a 0 when you want a 0, and you output high impedence when you want a one, rather than outputting 1. THis allows a 5V device to have output a 1 that is higher than 5V (more than the device can supply), all that is needed is for one end of the pull-up resistor to be connected to the line (obviously) and the other end is connected a voltage supply however high you need the 1 to be.

 

Impedance is a general term which may inclued capacitive or inductive reactance and resistance. It would be more accurate to say the 3rd state is high resistance because a frequency is not specified.

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So, if you were to turn on the third output, then a one nor a zero would show up? It would just be nothing?

 

Imagine a wire at ground potential, or at 5V. These are the two states we are familiar with. Now imagine the wire is disconnected from the rest of the circuit. That is the "tri" or 3rd state.

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It would show up as something...but what that something is, is unknown and unpredictable. It changes all the time and floats around, some ever changing voltage. It's basically "disconnected". Using a voltmeter to measure the voltage between power supply ground and a high impedence pin is simiilar to measuring the voltage between that same ground and some random piece of metal lying around somewhere.

 

Oh! I see now! Thanks! But then what is the purpose of the pin state? If it is so unstable, what is it good for?

 

You make the output pin of a device its disconnected 3rd state when you want to use the wire that its output is connected to by another IC's output.

Like I said before, the wire that the output pin is connected to can be used by many IC's that have 3-state outputs when only one output is enabled at a time.

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Uncle $crooge

 

YOu can use it for as an open collector output with the pull-up resistors so you can have a high voltage that is larger than the device's supply voltage.

And like audioguru said, if you have a Y connection between 3 pins, you can use the high-impedence to switch back and forth between one branch and the other. So you could conceivably have 3 or 4 uC using the same sensor over the same wire one after another. Similar to demultiplexing. (Right now, let's just say one output to several inputs instead of multiplexing which is several outputs to one inputs)