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0s and 1s

Posted on 1998-05-30
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  Why is it, that the nought is actualy representing 0.5v, rather than 0v ?
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Question by:crab1
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It's very difficult to get an electrical signal all the way down to 0V.  In practice, there are acceptable ranges of voltage that are interpreted as a 0 or a 1.  For standard 5V logic, a 1 is when the voltage is >2.0V at an input or >2.4V at an output.  The 0.4V difference is called the margin and allows for voltage drop in the interconnect.  A 0 is <0.8V at an input and <0.4V at an output.  Other logic families use different threshold voltages but the idea is the same.
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by:crab1
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 But, why do you need an electrical signal? - I think the answer is that it is difficult to react to a nothing voltage (ie. involving lots of circuitry, with not gates etc), whereas if you have a voltage, you instantly know that the signal is a negative one.
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by:jhance
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0V is not a nothing voltage, it's just 0V difference from some reference point.  For example, if I have to 9V batteries and hook their '-' terminals together, I can measure 9V between the "-" terminals and either "+" terminal.  If however, I measure from one "+" terminal to the other, I get 0V.  Both are said to be a the same potential so there is 0V between them.  It's easy to detect and react to a 0V situation in an electronic circuit, in fact, this is quite common.

Maybe you could clarify the situation you are asking about a bit more.  There is a fundamental difference between a 0V signal on a conductor and a "nothing" voltage.  Basically, there is no concept of a "nothing" voltage as a voltage is always relative to some other point.  
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BTW, when I said it's difficult to get a signal down to 0V, I meant that in the context of an electronic system having a 0V based power supply.  Something like +5V.  In this case, since the transistors used to pull the voltage down to the 0V power supply are imperfect, even under the best real-world circumstances, you don't ever get all the way to 0V.  If, however, we are operating using a split power supply (like +5V and -5V) then it's very easy to get 0V.  Now, of course, it's hard to get close to -5V.
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by:crab1
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  Yes, but what I originaly thought, (after being taught simple electronics in A-Level Physics), was that a 0 was 0v, and a 1 was Xv. The way this worked, was by having a circuit, with a switch ie, when the light is on, you get a 1, and when off, you have a 0. It seemed to me, therefore that it was unneeded to have voltage when giving a zero. But, I think, it is down to the handling of the message - If you ask a circuit something, and it returns a negative, with the original 0 = 0v, no answer will be recieved, unless you use etra circuitry ie. not gates etc, whereas with a 0 = .5v and a 1 = 5v, it is much simpler.
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  Hang on - surely the way to get zero volts, is to create a short circuit?! - Wait!, - I know this sounds bad, but that is exactly how a Not gate works, so surely the same applies to a computer?
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I think you're confusing 0V with a logic 0 and they are not necessarily the same thing.  The choice of which voltage in a digital electronic system is 0 and which is 1 is entirely arbitrary and chosen for the convenience of implementation.  For example, an older logic family used in many supercomputers is called ECL.  It used a -5.2V power supply and had a logic 1 at 0V and a logic 0 at -5V.  There is no special relationship between 0V and logic 0 in a digital system.
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by:crab1
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 Well, that was what the basic question was - why the logic 0 had to be represented by something other than 0v? Which as you say is because of the implementation.
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by:jhance
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Right, it's really up to the designer of the technology to choose what is a logical 0 and 1.  In what is commonly called "standard logic" because of it's widespread use, a logic 0 is around 0V and a logic one is higher than 2.0V.  Again, however, it's entirely arbitrary.
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