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Rob Philpott wrote: What sort of logic was this?
In computing - Three state logic.
In formal logic there is also ternary logic, in one scheme:
+1: True
0: Unknown/ Indeterminate
-1: False
You've probably already used this without realising, nullable bit fields in SQL work along ternary logic lines.
Alberto Brandolini: The amount of energy necessary to refute bullshit is an order of magnitude bigger than to produce it.
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Certainly, at a software level. But I've yet to see any sort of bus that uses three logic levels.
Regards,
Rob Philpott.
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Look to FPGA's (Lattice) for that, I have worked on some Tri-State video busses (migraine guaranteed!) 
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the 205 from Kings Cross to Paddington via the Mall?
[edit] to put correct bus number before I was corrected[/edit]
You cant outrun the world, but there is no harm in getting a head start
Real stupidity beats artificial intelligence every time.
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Bergholt Stuttley Johnson wrote: [edit] to put correct bus number before I was corrected[/edit]
I try to avoid buses as they're always full of 'them', so have no ideas about numbers.
That said, bearing in mind all you have to do is follow the Euston Road to make such a journey it surprises me that you take The Mall in on the way.
Either way, I'm reasonably sure that they don't use -5v.
Regards,
Rob Philpott.
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24v 64hp London Omnibus
You cant outrun the world, but there is no harm in getting a head start
Real stupidity beats artificial intelligence every time.
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Rob Philpott wrote: Certainly, at a software level. But I've yet to see any sort of bus that uses three logic levels.
Check out what a qubit is for quantum_computers[^]. It's the future of tech, at a really early stage though.
Jeremy Falcon
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in many case three state of currency doesn't mean three state of logic.
The -5 / 0 / +5 differentiation is used just for electrical reason.
If use 0 / +5 you can have a sort of eco in the signal wich transform it self into noise.
With negative voltage there is a resorption of this eco but the chipset will not see the zero.
Practically is -5 for false and +5 for true
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Keith Barrow wrote: In computing - Three state logic.
Yeah, it's technically possible, but indeterminate boolean values make my skin crawl. They aren't even boolean, really.
Keith Barrow wrote: You've probably already used this without realising, nullable bit fields in SQL work along ternary logic lines.
And this is exactly why I hate them. You can have nullable bit fields, but you can also hit the server with a hammer, that doesn't make it a good idea. I'm sure there are some cases out there where this is useful, but I've only found it to be problematic. Every time I find nullable bit fields in a database, it's an issue that needs fixing rather than some clever use of three-state logic. If you need more than two values, why even use a bit field for that? Integers will do fine, and you won't have to deal with nulls.
Keith Barrow wrote: In formal logic there is also ternary logic, in one scheme:
+1: True
0: Unknown/ Indeterminate
-1: False
Yeah, but in general classical logics don't allow indeterminate values. True, false, or GTFO.
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Rob Philpott wrote: -5v? I can't see how you can have tristate binary. What sort of logic was this?
Quantum computers use tri-state binary.
Jeremy Falcon
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Yeah, perhaps out of scope of the original question.
Quantum computers do my head in. Witchcraft, I tell you!
Regards,
Rob Philpott.
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I thought it was just a complex superposition of 2 states, which ends up giving you 3 dimensions to work in so you can get things like 30 degrees away from true. 
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Well, I'm still fuzzy on all of it, but from what I gather that's what it's intended to represent. But it still has to be stored somehow, the third state is a "indeterminate" flag to say it could be anything. But, it's still three states stored in the system.
Jeremy Falcon
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Chuck Norris has a three-state one-two punch.
You'll never get very far if all you do is follow instructions.
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There is a problem with ternary logic.
"Bit" is short for "binary digit".
What would you shorten "ternary digit" to?
The problem is equally terminological as technical!
PS. In the UK, I am aware that a tit is a kind of bird.
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Quote:
PS. In the UK, I am aware that a tit is a kind of bird.
Amongst other things yes. 
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Vivic wrote: tit is a kind of bird
Nah, it's what birds have two of.
You'll never get very far if all you do is follow instructions.
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That is true too. You just phrased it very well. I didn't figure out how to use 'bird' meaning 'girl'!
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Vivic wrote: "Bit" is short for "binary digit".
What would you shorten "ternary digit" to?
"tet" of course! 
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
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I find that offensive.
You'll never get very far if all you do is follow instructions.
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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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