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Rob Philpott wrote: -5v? I can't see how you can have tristate binary. What sort of logic was this?
Its a matter of interpretation.
The physical characteristics are represented by a tristate value while the actual usage relies on two state values.
A more esoteric example is in RS 232 protocols where 1 and 0 are represented by how the slope of the electrical signal is moving.
You can google for the following to see pictorial representations of that.
rs232 falling edge
That said there is a idiom (or 'theory' maybe about 3 state logic) which is discussed in the following
http://en.wikipedia.org/wiki/Three-valued_logic[^]
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That was some pretty good info. Thanks 
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As Ryan said it's all to do with the hardware, 0 or false tends to be a voltage value, it can be plus or minus depending on what is around it, due to the individual 'closeness' of conductors that are insulated from each other acting like capacitors. The actual what is 0 can be determined by looking at the data sheets. Very rarely is 0 actual 0 there tends to be an over lap due to tolerances in the components used on the board. RS232 gives a good version of this have a look at my article on serial comms it explains it.
Glenn
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The answer is it depends. The signals sent are either RZ (Return to Zero Volts - I assume with a small error) or NRZ (Non-Return to Zero volts - a small voltage). I have no idea which the main in a typical PC bus uses, I assume RZ as it consumes less power and there is no need to detect if the signal is active.
[Edit]
See glenPattonPUB's reply - he knows what he's talking about.
Alberto Brandolini: The amount of energy necessary to refute bullshit is an order of magnitude bigger than to produce it.
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Why, thanks kind sir!,[Yosser Mode] Gissa a job, electronics, I can do that![/Yosser Mode] 
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You can check the spec sheets for processors, generally these days they run on less than 5V, more like 3.3V or less. A range 0V through 1.5V will be treated as 0, and 1.5V to 3.3V will be treated as a 1.
Docs like this
[^]
will help (table 3 - min high level, max low level)
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Both 0 and 1 in a chip are usually represented by a voltage rather than a current, because they're made out of FETs instead of BJTs.
Anyway, a closed transistor doesn't reach infinite resistance, not even the silicon oxide around it has infinite resistance, so you'll always have some non-zero voltage (and current).
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Usually the logical levels are represented by voltages, not currents. You may have either active-higth or active-low signals, see Logic Level at Wikipedia[^].
THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?!
-- C++ FQA Lite
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CPallini wrote: voltages, not currents
Yes, but can you have one without the other?
You'll never get very far if all you do is follow instructions.
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Current Flows, Voltage is Pressure to use the water metaphor...
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Yes, I know that, but it doesn't answer the question.
You'll never get very far if all you do is follow instructions.
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Well Voltage 'dropped' across a component, it appears one side, a different value on the other side giving a difference in voltage that can be read with a meter in parallel. Current is the 'thing' (for want of a better word) moving through the circuit to measure it you have to break the path and insert the meter (there are other methods like coils etc. but...). So you can measure a voltage across something with out the being current flow (such as wires before plugging in). Bad explanation but the best I can manage at this time-O-day have a look at howstuffworks.com I seem to remember they had a very basic explanation. I'm off home!
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That is not the point.
Logical levels are bound to voltages because electrical potential difference is the measured physical quantity.
For instance a car battery providing 8A to the vehicle lamps would have no higher logical level than four CR2032 cells providing 20 mA to a red LED (and its series resistor).
THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?!
-- C++ FQA Lite
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I know that, but it doesn't answer the question.
You'll never get very far if all you do is follow instructions.
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Yes, othrewise I wouldn't ask the question.
You'll never get very far if all you do is follow instructions.
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To answer your exact question: no.
THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?!
-- C++ FQA Lite
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 Exactly.
You'll never get very far if all you do is follow instructions.
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The closest that you could get is a battery hooked up to a capacitor. A capacitor is just a gap in the circuit with a large surface area. Electrons pile up on one side and the force they exert on the other side repels the electrons on that side. Of course there is always leakage current in real life, and things get a little more complicated with A/C.
http://en.wikipedia.org/wiki/Capacitor[^]
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Bits can be stored in many many forms . They can be in the polarisation of light , in magnetic state , by the presence or absence of a stored charge . But in electronics the information is usually transmitted as the presence or absence of a voltage . There is usually a very small current associated with that , we don't want a big current . The values of the voltages used and the corresponding currents are somewhat arbitrary . Some systems represent a 1 as the presence of a specific positive voltage ( typically 3 or 5) and some use 0 volts to represent a boolean 0 . This is not mandatory . It is equally possible to represent a 1 as 0 volts and a 0 as (say) 12volts . And if your system is noisy then some systems will use a positive voltage to represent 1 and a negative voltage to represent 0 . There is no hard and fast rule . Although in most systems +5v => 1 and 0v =>0 .
There can never be an exact 5v or an exact 0v , you will always have noise induced in the circuit from a wide variety of sources . But the entire reason why binary systems are used is that they are able to handle the noise very well . When the noise voltages are in the millivolt range then it is going to take a lot of noise/bad luck for the noise to be so large that we cannot recognise a 1 and a 0 when using 5v systems . BUT it can and does happen, and it gets worse as the data frequency increases . We do get noise and some technologies are more prone than others . In this case we typicaly add extra bits on the side in order to help . These can be simple parity bits or more complex systems where not only can we detect 1 or 2 bit errors but correct them too.
There is nothing stopping you having a system where we use 3 , 4 more voltage levels . And in that way we could have a single line carrying more than one bit of information . But such a system would be more susceptible to noise . In order to reduce the effect of noise you would probably have to slow the data rate .
But you do not have to use simple voltage to transmit information you can do all sorts of fancy manipulation of the frequency and phase of a signal , and these are used in broadband . These can have many possible states , but are also less susceptible to noise , enabling the continued growth in the broad band speeds .
The whole area is fascinating , if you enjoy electronics. But if you don't enjoy electronics and then its probably easier to think that yes 1 is 5v and 0 is 0v and live in blissful ignorance of what is actually happening several billion times a second in your humble pc or phone.
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Quote: The whole area is fascinating , if you enjoy electronics. But if you don't enjoy electronics and then its probably easier to think that yes 1 is 5v and 0 is 0v and live in blissful ignorance of what is actually happening several billion times a second in your humble pc or phone.
Good explanation, I have a feeling that will not be the end though! It fried my brain to think of a one as -5v and a 0 as +5v when I started many moons ago!
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In a -5 to 5 volt system, minus values are treated the same as positive. the 0 is in between 0 and 0.8 and the 1 is in between 2.5 volts and 5.
Panasonic developed three state chips which read 0.8 to 2.5 volts as a third value.
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I always thought zero was 1070/2025 Hz and 1 was 1270/2225 Hz (that is up/down).
At least that is how 0 and 1 was represented when I learned my first BASIC programming. Oh well, that was way back in late 1975. Maybe newer implementations use different representations.
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Every circuit has noise and dark current, both of which tend to increase with temperature. Whatever the circuit treats as a logic 0 or 1 is within a certain tolerance.
Marc
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