Can anyone recommend a tutorial on how CANBUS works? Not really Jaguar specific just in general. I have read one or two I found via Google but they seem to concentrate on the networking aspect which I get as I have a computer background, I'm more interested in the way it's used to control and communicate in real applications - I suspect like the way the wing mirror will dip to help parking if you are in reverse and under 12 kph (I think).

Mike

 

drmike, if I remember things correctly, you have to start with thinking of a loop of data. The points looked at are in the same order time after time after time. This loop of data then runs through every computer on the bus, making many, many loops a second (the number depends on the amount of data and the speed of the slowest computer). As a data point comes by, the computer it is going through goes "is this a bit of data that I need to fill in or use? If no, it lets that value remain and keeps passing it on. If yes, it takes its last value and puts it in the loop or notes the value and puts it into the registry of the local computer. After 1 loop, all the values should have been updated. In addition, there are a few check values put in that say the computer turns a 1 to say "yes, I am alive" and when it goes to a later computer, it gets turned back to a 0 You then have a computer between the two that is looking for a 1 to say all is good and if it sees a 0, it throws up an error code after a number of failed checks in a row.

With the data updated and the registries filled with the latest data, the computers can then process this data and perform the necessary actions or create the needed values to calculate other items.

 

Data goes past each module, visible by each - ignored if for other(s).

Not a loop, think of a string of modules, 60ohm resistor at each end (conceptually, at least).

That's why a module can be removed without grief if it's not vital.

Two wires (plus ground) with the voltage difference signalling a 0 or 1. Packets of data with checksums etc. Each packet with ID for who to & from.

Far more on web sites

 

Thank you both.

So are you suggesting only the modules can create and modify the packets of data on the bus or are there other bus nodes that can do this as well? It would certainly be possible to create a simple PCB that could post/update data on the bus but it might be simpler if each module is wired to say the window PCB where relevant.

 

Module = node. Same thing.

Packets don't go through a module (*) so can't be altered.

(*) except with more than one bus (used to be very rare), when the "bridge" aka "gateway" might.

A DIYer might add such a gateway e.g. to watch packets and alter or add them.

(Or just add a node, such as an OBD device.)

Jaguar do use such gateways...

More than one car maker has joined the vital CAN to e.g. music/etc CAN and allowed hackers to get from music/etc to vital & take control e.g. of brakes!! Yes - that bad at system design!

 

Module = node. Same thing.

So only the modules generate packets, there are no subsystems that generate packets.

 

What? No idea what you mean.

 

I'm sorry I didn't express myself well!

I think you're saying that the only CANBUS nodes are the control units - is that right?

 

Call them node, module or control unit, whichever you prefer.

 

drMike,
The best CAN training videos that I have found are by Kvaser trainer Bryan Hennessy.
The first video is:
Check the Kvaser U-tube video index for additional videos.

If you intend to start a CAN study group, count me in!

best regards,
Bill

 

Also see:
https://www.kvaser.com/course/can-protocol-tutorial/

 

Thank you. I met a guy who lives in my village who works for what remains of Lucas (yes I know all the US thinks Lucas is useless but in fact they were highly innovative) having been bought by Rolls Royce and he has some knowledge of CANBUS so I'm moving forward.

 

It's a LOT about low level stuff you're very unlikely to need.

 

Kvaser does have a LOT of information available. I'm still studying CAN too, so your tutorial suggestions are welcome.

Bill

 

Adam Varga has a much more practical series of tutorials.
The first is A discussion on this level would be welcome.

Bill

 

Thank you!