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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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In a lot of chips I've worked with there is an internal threshold (sometimes external resistor) that if the voltage is above "X" it is on, if below it is off.
Serial ports work like this with either a 5vdc reference or a 2.5vdc reference and then is split down the middle. there is an effect called field magnetism (bleed)where other external voltages can influence (and will) the wire voltage so you never get a clean 0vdc. So chip manufactures tend to build this into the chips.
a signal could look like: 4.5, 1.2, 3.8, 0.6, (on,off,on,off) and still be perfectly valid, that's where oscilloscopes come in handy for troubleshooting. 
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The reality is that all circuits are really analog, they don't instantly change from one state to another.
So, the transition is never at one extreme or the other. If the chip claims to work at say 0 and +5 there is some point before +5 that the circuit decides the signal is a 1 and some point before 0 that it decides that the signal is a 0 (replace 0 and +5 with your favorite voltages for representing 1 and 0.)
In diagrams they like to draw straight lines and sharp corners but the reality that none of the lines are straight and none of the corners or sharp. In fact, there is usually some amount of overshoot at the corners and a certain amount of settle time. They have a couple of pictures here of what overshoot looks like.
So, if they used the 0 or the +5 for the actual 1 and 0 indicator it would get very tricky because the signal (at least at the corners) tends to bound around the min and the max voltage...
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Not sure what you mean by currency. You generally have voltage, current, and resistance.
As for 0 and 1, generally this is a measure of voltage only. And most of the time 0 is near ground potential and 1 is near Vcc (the voltage common to the circuit). So if your wondering if there might be some amount of residual voltage - yes. In fact it with a typical circuit, most of the time the pins will not be exactly or Vcc. They will be slightly higher than 0 or slightly lower then Vcc. And occasionally, you can actually et something that is very close to half way between, but such a situation is generally a result of some problem in the circuit.
Having said all this, in most cases in working with a digital circuit it helps to set the scale and simply lance at the value. High and low will be very obvious.
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I saw a lot of good answers here, but wanted to point out that there are some very common devices in personal computer architecture that use multiple voltage levels to represent more data - MLC solid state devices, one example of which is the Triple Level Cell SSD from Samsung. These use 8 voltage levels to represent 8 bits of information per cell (as far as I remember). The details have gotten a little hazy, but this^] article explains it pretty well. In other words, 0 is only represented by the accepted range of the voltage present, as any other value would be.
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Algerian Airlines this time....does anyone else think Nagy should fly to Hungary instead of taking the car? 
Seriously though is anyone going to fly in the next few days it would be squeaky bum time wouldn't it?
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I would prefer to fly, but the cost for four with luggage and a hire car when we get there is stupidly expensive compared to the tunnel & 6 tanks of fuel.
The time saved is actually negligible, when you consider door to door is close to ten hours flying to get down to Fured.
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Nagy Vilmos wrote: I would prefer to fly,
I love to drive across Europe, the furthest I've done is Salou, not sure how that compares distance wise with where you're going.
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I just chickened and it is about the same distance +/- a dead fox or two.
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Do you stop on the way? Sleep in the car or get a hotel?
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We almost go straight through. We'll cross over and stay the night with friends in Brussels, then tomorrow we'll depart around 6 and should be on the beach before sunset. 
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Quote: The time saved is actually negligible,
Really? but you do have to cross over on the wrong side o' the road? , Quote: Fured?
do you mean Balatonfured...
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