Multiplexing / CAN

PowerLee · Post by **PowerLee** » 20 May 2006, 10:59

Hi

Just been learning about multiplexing / CAN on a training course.

All I can say is what a wonderful system CAN is.

Once it has been explained to you how it works, How easy it is to test with a multimeter & all the benefits CAN gives you I can honestly say I will never have a non CAN car again.

I thought learning about CAN would be a total nightmare & headache, But after it was explained to me & I had seen & heard it in action I am total in awe how wonderfully simple it all is & what benefits it gives to us to.

Thanks Bosch for giving us CAN.

fastandfurryous · Post by **fastandfurryous** » 20 May 2006, 11:13

Is all this information in the public domain though? So that people like me, who refuse to go anywhere near a dealer/garage, can fix their own CAN equipped car?

PowerLee · Post by **PowerLee** » 20 May 2006, 11:38

Im sure a search on the internet will throw up all the CAN information you need.

I even had a look at the 206 CAN wiring diagrams for my car on the internet yesterday, Very simple to follow

Basically CAN is 4 wires, + & - as well as CAN high & CAN low

If you suspect a break in the CAN loom you can test it with a multimeter.

You should find you have + & - on the 2 supply wires.

There's a 120 ohm resistor at each end of the CAN loom across the CAN high & low wires to help fault find breaks in the loom.

Then anywhere across the CAN high & low wires you should find around 60 ohms resistance when you test with your multimeter if the CAN loom is intact.

If those 2 test are ok then your CAN loom is intact & the problem lies elsewhere.

Also CAN sends micropluses down the CAN wires many times a second to everything that's connected, If all is ok then the CAN system is happy.

If somethings disconnected then you get a fault code reported.

I really thought CAN would drive me mad, But seeing it in practice & having a play testing it I find it so simple to work on.

Clogzz · Post by **Clogzz** » 20 May 2006, 12:34

Looks familiar.
This system has been used in electronics for 30 years.
It’s microprocessor control, where you have a supply and earth, a ‘data’ line and a ‘clocking’ line.
Clock and data are trains of bar codes.
In case of difficulties with old cars, where say a tail lamp doesn’t work because its electronics don’t unscramble the bar codes, you add a wire and switch to light it, always ensuring not to tap into the bar code wires.
That system in itself has not been an area of problems.
It does mean though that you will need an oscilloscope to see if both bar codes properly arrive at the lamp’s electronics when it doesn’t light.

I can see plenty of new wires and bodgies coming.

When they arrived with multiplexing, they told us that it would reduce the number of wires.
Instead, it looks like it’s now 3 wires where there was only one.

mbunting · Post by **mbunting** » 20 May 2006, 16:51

Yeah, but the beauty about this CAN stuff ( as far as I understand ), is that you would only have the 4 wires going around the car, with devices plugged into it - like the network at home/work.

Messages are sent around this network with a source and destination type tag, so when you press the "window down" button, the box of tricks behind the button codes that into a message, which gets sent onto the network, and read by the window motor controller ( because the message is titled WINDOWS and says lower window 1 ).

This makes it much easier for systems in the car to talk to each other, and read data - for example - the auto wiper sensor detects that it's raining, and sends a message. The wiper controler hears "it's raining", and activates the wipers. The window controler hears the same message, and ( if configured ), closes the roof / windows etc, and activates other things ( like the previous e-class brake-by-wire system that periodically lightly applied the brakes in the rain to keep the discs nearly dry ). The speed sensor would talk to the abs, esp, perhaps even an economy module to wind the windows up and turn the aircon on ( to improve economy ) etc..

Great in theory, but do the control modules weigh more than the old looms......

pugxpert · Post by **pugxpert** » 20 May 2006, 21:17

i would agree with clogzz there,had a few 307's where the mobile phone man has chopped into the data wires it causes havoc. powerlee have a look at www.peugeotmt.co.uk some can bus data wire scope traces on there.good example of a faulty one on a 307 with water in the bsi

fastandfurryous · Post by **fastandfurryous** » 20 May 2006, 23:57

My concern with a fully integrated system like this is that a fault in one sensor/system can cause havoc with the whole car.

Like the problems with 406's getting water in their CD player, that then causes problems with just about every other system in the car!

It has to be better for robustness to have discreet systems working independently of each other. Integration is cheap, and hence that's what's done.

Clogzz · Post by **Clogzz** » 21 May 2006, 11:57

And that’s what’s kept me off the streets for over 30 years.

Latest stunt was a large LCD TV costing thousands of $$, where the digitally controlled surround expander IC packed up and shorted both data and clocking lines to earth.
The lot stopped, impossible to turn it back on, there was just the stand-by light.
There are several processors, but they’re all linked by the data and clock lines, so absolutely nothing could work.
And the fault should have just been loss of surround expansion.
Nice outlook !

pugxpert · Post by **pugxpert** » 21 May 2006, 18:52

yeah i had one of the first 406's back in 99 with water in cd player that took some finding,

Homer · Post by **Homer** » 21 May 2006, 20:36

PowerLee wrote:
Basically CAN is 4 wires, + & - as well as CAN high & CAN low

Someone dscribed to me it as being similar to USB.

USB has a +5v, Ground, +data and -data

PowerLee · Post by **PowerLee** » 22 May 2006, 07:22

Homer wrote:
PowerLee wrote:
Basically CAN is 4 wires, + & - as well as CAN high & CAN low

Someone dscribed to me it as being similar to USB.

USB has a +5v, Ground, +data and -data

Same difference

Clogzz · Post by **Clogzz** » 22 May 2006, 12:11

the mobile phone man has chopped into the data wires

Aren’t the data wires thin and the current wire thick ?
The can/can wires don’t carry current, so I would expect them to be thinner to split a few pennies.

pugxpert · Post by **pugxpert** » 22 May 2006, 13:36

data wires are useally twisted together to stop interference

ACTIVE8 · Post by **ACTIVE8** » 23 May 2006, 09:04

A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. When electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect. Using cancellation together with twisting the wires, cable designers can effectively provide self-shielding for wire pairs within the network.

fastandfurryous · Post by **fastandfurryous** » 23 May 2006, 11:33

The only reason we have twisted pair data cables is because it's cheaper and easier than using Co-Axial cable.

Yet another instance of doing something cheaply rather than properly.