wiring

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crooser
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wiring

Post by crooser »

i am puzzled.what exactly is,multiplex wiring?
Kered
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Post by Kered »

Lets look at powering up a tail light in a normal wiring system. A wire from the battery goes to the fuse box. The fuse is an appropriate size for the total load of tail lights and side lights. A wire goes from the fuse to the light switch. A wire goes from this switch to both tail lights and also to both sidelights. This entails a lot of wire running all over the place - and this is just for tail lights and side lights. Think of all of the other electrical items that need switching - brake lights, indicators, heated rear window, mirrors, windows, fans, motors - it's endless. Every item needs a wire from it's own fuse to the controlled item via its controlling switch. The more current taken by the item (such as the heated rear window) then the thicker the wire from the fuse via the controlling switch to the item. A lot of wire - and heavy.
Is there an alternative? Well that's what multiplexed wiring offers. It's based on the way computers work but I'll try and keep it manageable but if I cause confusion then please shout and I'll have another try!
Some items need a heavy current supply and some need little. Why not just feed a pair of thick 12V power wires round the car suitable for every item to take its power from? Sounds OK but how do you switch the tail light on or the heated rear window?
This is where the computer bit comes in. Each item that needs access to these power wires has a "remote switch" (which is physically near to the switched item) and connects, say our tail light, to these power wires when the normal tail light is switched on. Same with the heated rear window. Switch the heated rear window switch on and the "remote switch" will connect the heated rear window to the thick 12V power wires.
What's the advantage? Well Citroen only need very thin wires from the driver operated switches to the remote switch which then takes the power from the thick power wires which will run round the car and will never be far from anything that needs power.
In fact Citroen not only uses thin wires but can control anything from these same thin wires that also run round the car with the thick power wires. The signals on the thin wires are coded so that the same thin wires control every remote switch.
Terribly confusing I know! Let's say the tail lights are coded 3 and the heated rear window is 9. Operate the switch for the tailights and the code "3" will appear on the thin wires, Only the remote tail light switch will recognise this and switch them on. Similarly if a "9" appears then only the heated rear window will recognise this and switch on.
It's called addressing. Every item controlled has it's own address and when the instruction arrives at an address then the item living at that address will recognise it and know what to do with the information.
To the user it works as it should. To Citroen it saves weight, complex wiring and money. Is there a downside? Well fault finding will be much more difficult as there is no direct connection from the operating switch to the item being operated. Happily, so far, it all seems reliable!
Derek
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Post by John F »

Another advantage being that the distance between the heavy duty "ring main" and the unit being operated is much shorter so that voltage drop is greatly reduced over the short length and wiring overheating is reduced.
crooser
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Post by crooser »

thanks lads,i now understand the principle but i would hate to try and trace a problem using normal methods.
pwatson
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Post by pwatson »

I am very thick to say the least when it comes to electrics, but can you explain a bit more about the "thin wires". I understand the ring main bit but is there just one thin wire from all driver's switches to the remote switches which carries all the signals or is there a thin wire from each driver's switch to each remote switch and if there is only one how does it carry several signals? (say for rear lights, brake lights and indicators which may all operate at the same time?)
Thanks
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Post by NiSk »

No, there's a "power" bus and an "signal" bus. The control units communicate in a similar fashion to computers in a network - they only need two thin wires (he says, greatly simplifying everything).
//NiSk
Kered
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Post by Kered »

The term "multiplex" is often used these days and means different things for different purposes. The overall use of the name is for one "thing" to carry different strands of information. For instance in a multiplex cinema one enters one building which houses several sub units with different information being available all at the same time. In UK terrestial digital TV (Freeview) one transmission frequency carries several channels and is referred to as a multiplex.
The same happens on Sky satellite TV. The important thing is that all of the different strands of information are available all at the same time.
Obviously it can't really all happen at the same time (except the multiplex cinema) but the mixed information travels much faster than its end use where it then gets reconstructed. It is so well done that the end user is none the wiser. The same happens to telephone calls and you have no idea of it.
So in cars one set of data wires can be coded in various ways to activate all of the items that need switching - apparently all at the same time if needed.
Shout if still not clear!
Derek
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Post by pwatson »

So presumeably the multiplex looms are now much simpler and less complex (in appearance if not operation)? - well, simpler than the wiring I removed from an old BX I scrapped anyway which covered the whole of my back lawn and weighed a ton!!
Thanks for the explanation Kered - all is now clear!!!!!........but how does electricity work ?????!!!!!!!
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Post by bernie »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by pwatson</i>

but how does electricity work ?????!!!!!!!
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Put a shilling in the meter[:D][:D][:D]
NiSk
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Post by NiSk »

Whats that in €??
//NiSk
Kered
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Post by Kered »

Since when was Sweden in the € ?
Derek
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Post by AWG »

I like this place: decent explanations of the technology with insight into political economics to boot.
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Post by r_306_XSi »

Can I just aska geeky question (having been in the electronics industry for 10 years!) - does anyone know if the multiplex wiring described in this topic is also known as CAN Bus?
NiSk
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Post by NiSk »

Controller Area Network Bus is the most common form of multiplexing on cars. It's a standard approved by SAE and ISO. Read more about it on:
http://www.canopen.us/history.htm
//NiSk
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Post by uhn113x »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Thanks for the explanation Kered - all is now clear!!!!!........but how does electricity work ?????!!!!!!!<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
It works by smoke:
Electricity is driven by electrically pumped smoke. It is atomized, and then sent down the wires an atom at a time. If anything fails, when you turn the power on again, the smoke leaks out. You don't see smoke if the power is off (unless there is also a fire, but this only happens in bad sci-fi movies, when control panels unexpectedly burst into a shower of sparks (smoke moving at light speed) and flames).
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