In message <
[email protected]>
Erik-Jan Geniets <
[email protected]> wrote:
>
>
> Austin Shackles wrote:
> >
> > On or around Wed, 23 Feb 2005 13:42:03 +0100, Erik-Jan Geniets
> > <[email protected]> enlightened us thusly:
> >
> > >
> > >
> > >beamendsltd wrote:
> > >
> > >> Citroen had a all CAN bus version of the XM (not available
> > >
> > >What is a 'CAN bus'?
> > >Erik-Jan.
> >
> > used on one of the modern Range Rovers, is it the P38 or the new one?
> >
> > instead of wires running all over the vehicle carrying current, you have a
> > power supply, earth and data connection to (e.g.) yer rear light cluster,
> > and when you want the rear lights on, instead of switching the current and
> > sending it down a wire, the BECM or equivalent sends a message to the rear
> > light cluster telling it to switch the rearlight on.
> >
> > OWTTE...
> >
> > has advantages, but then again, it's far less easily fixed.
> Thanks.
> So at has some sort of relays all over the car?
> Erik-Jan.
Ah! My favourite subject! I was Feature Owner for the CAN bus
on the "Medium Size Bentley".....
The great advantage of CAN (or VAN etc) Bus systems is that
it removes the need for all those nasty wires (the harness)
runing round the vehicle, thus reducing design complexity,
vastly increasing realiability and flexibility. Bearing
in mind the harness is one of the most expensive, heavy
and (to an extent) unreliable components of a vehicle, then:
The entire harness can be done away with and replaced by
four wires that visit each "electrical point" in the vehicle.
This uses standard connectors, standard cable, wieghs
almost nothing and costs buggger all. However, each electrical
point increases in complexity - a bit. Although we had a Jag
doing just this about 8 years ago, no one has *fully*
implemented yet - there's still a lot of cable about.
So, for example, the rear light cluster. A message is
broadcast from the Body Controller ECU (could be straight
from the brake light switch if it had a bus controller).
A standard IC in the rear light cluster spots the message
and says "Ah ha! Thats for me" and then decodes the contents
of the message.... "switch on the brake light on" - which it
does. So far it's no different to cable, but now the good
stuff kicks in. The brake light bulb is blown. The light
cluster controller (which is electronic, no relays) can now
do things like :
Use the tail light instead if the lights are not on, or flash
the tail light rapidly as a back up if the lights are on.
It also broadcasts a message telling the rest of the vehicle
ECU's (if they are interested) that the failure has occured.
Most likely in this case, the Body Controller would tell the
driver that the bulb (or more than a given percentage of LED's)
has failed, and could then log the fault for the next service
or whatever. If an ECU "disappears" from the bus through damage
or something, then the message originator can re-try for a while,
and getting no response, tell other ECU's to act accordingly.
If you extrapolate this functionality throughout the vehicle,
then the possibilities are endless. When the car is built, the
ECU's attached to the system can determine automatically what
features are attached to the vehicle - no different harnesses
for different spec vehicles. Upgrades can be truly plug-and-play,
or even enabled using a code when the extra dosh is handed over.
If the keys have different codes, then as well as seats,
mirrors etc being moved for that person, the performance of
the vehicle can be modified - e.g. Daddy can have full blown
sports mode, young Rebecca can have much reduced performance to
save on insurance - all with no extra cable.
You can log all this to give info leading up to an accident -
even who's key was being used - if Rebecca was using Daddy's
key then the insurance could be invalidated etc.
Compared to the above, the tradition harness doesn't even get
a look in! We had an entire Jag on a 6x4ft bit of chipboard
for demo purposes (as well as a real car!) - the real cars
harness would have taken (at least) two of us to carry it.
Personaly, I drive a 200Tdi 110 - not an ECU in sight. but then
I don't want fancy toys anyway......
There are two flavours of CAN - high-speed and low-speed, the
high speed flavour is for engine management etc, and low-speed
for lower demand systems. It is an amazingly robust system,
originaly designed for machine and factory automation and
remote control. There were other contenders (VAN being one),
but unless things have changed recntly, CAN has won the day.
Richard
--
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