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(Post Link) post #9 of Old 1 Week Ago
jamesinnewcastl
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Join Date: Jun 2009
Location: United Kingdom
County: Tyne and Wear
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Quote:
Originally Posted by alfaitalia View Post
Not quite true....it also decides where and whether to send that data...for instance on some cars if the seat belts are not engaged it wont send the signal to release the electronic handbrake...and lots of other similar linked actions. It also changes the signal to and from a CAN bus data signal to a CAN bus controller signal. Receiving: it converts the data stream from CANbus levels to levels that the CAN controller uses. Transmitting: it converts the data stream from the CAN controller to CANbus levels. So if you are expecting you mic data to pass through the canbus unchanged (if it even goes through it....I have my doubts) I dont think it will be. Unlike you two Im most certainly only a keen amateur....so I happy to be proven wrong!!!!
The CAN Bus doesn't decide anything.

Imagine Churchill, Roosevelt and the three Armed forces Chiefs in WWII, they all had to pass information to each other and receive information from others in order to make decisions and order things to happen. There was also a postman. While the war was executed by all the famous people they needed the postman or it wouldn't have happened. Now Postie had no idea what the others were saying in the letters but he had rules, he would go to a standard post box where people had put the letters, collect the standard envelopes with an address on and deliver it to another standard post box.

The CAN bus is a postman, you give it information via a standard interface and it sends it, interested parties pick it up by a standard interface and use it. (OK that's more like Ethernet, CAN is a little different but lets not get too deep) The interfaces and the bus have to be standard because companies in the future might want to connect new equipment to the bus and they need to be sure that they will be accepted by the postman (because he is a simpleton and only knows dat rules is rules.... seriously computers are thick, but fast)

What you state about levels etc is true - but that part is standard and all kit will do the same, importantly it does not include any decision making. In your seat belt situation it is actually Winston Churchill who wont let Roosevelt release the handbrake unless the RAF says it's OK, postie just delivers the letters between them. Unlike simple electrical circuits where everything happens instantly the software in the control units can monitor other equipment over time and only make a decision if it 'thinks' it is safe (for example). Even then that control unit could decide to flash a light and wait for the driver to press a steering wheel key before releasing the handbrake.

This is why systems with software in are so hard to fathom - you can't tell what they might do just from observing them or having a wiring diagram, you have to be told what they will do. What they do is exactly what the programmer wanted them to do, so you have to ask him. So if there is no documentation you may as well cast some goat entrails....

Incidentally, it seems that audio can be sent over CAN, just google it. I suspect that the audio would be compressed to avoid hogging the bandwidth. Again this reflects the fact that CAN is just a postie and doesn't give a monkeys what you are sending. (Apologies to Postmen - you are essential!)

I can't find any block diagrams of the CAN system myself or I could be more useful!

Cheers
James

Last edited by jamesinnewcastl; 1 Week Ago at 12:18.
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