questions regarding CAN transceivers

Hi there - I've been using CAN to connect a number of boards together. Initially, the entire system was 3.3V, so I used TI sn65hvd231q parts for CAN transceivers

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sn65hvd231q.html). Later on, I had to switch some of the boards to a

5V supply, so on those, I switched to using TI sn65hvd251 CAN transceivers
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sn65hvd251.html). Odd thing was - suddenly I lost communication to the boards running the 3.3V parts. All of the 5V boards could talk to each other just fine, however. I was able to quickly reestablish communication by switching over to a sn65hvd251 on the 3.3V boards (though I had to also supply 5V to just the CAN transceivers - which was a pain).

Anyways - why did I lose connectivity here?

Also - can anybody explain to me what exactly the functionality of a CAN transceiver is? I guess I've never fully understood what they do.

Also, do CAN controllers ever come with CAN transceivers integrated? To me that would be incredibly helpful as most of my CAN enabled boards that I've worked on have been very limited in terms of board space - so eliminating extra chips would be awesome.

Thanks!

-Michael

Reply to
Michael
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CAN has a dominant state (logic 0) and a recessive state (logic 1). A 0 can overwrite a 1, but not vice versa.

The recessive state is both CANH and CANL being at about 3V. In the dominant state CANL is pulled down by about 0.5V and CANH is pulled up by about 0.5V, so CANL=2.5V and CANH=3.5V. With only 3.3V supply you don't reach the high level reliably.

The lesson you've just learned is: Only use CAN with 5V. Period.

The transceiver is a level converter between standard digital CMOS levels and CAN bus levels.

No, never. Digital and analogue mfg processed don't mix very well, and there are also other ways to transmit CAN - by POF (optical) or only with a single wire. These busses need different transceivers. And you should choose different transceivers for high speed CAN busses (500-1000 Mbps) and for slow CAN busses (=

Reply to
Frank-Christian Kruegel

Maybe should it read high-speed CAN busses (500-1000 Kbps) and slow CAN busses (=< 125 Kbps)...

Regards, D.

Reply to
D.

an

ou

Well - how can these parts be marketed if they aren't functional? Furthermore - they were working perfectly when it was entirely a 3.3V system.

ere

gle

I really need to run it at full speed, so is the sn65hvd251 a good choice for that? So far I have had no problems whatsoever with it, but it hasn't been stress tested as much as it should just yet.

Also - what optical solutions are there out there? Can you achieve full speed optically with CAN?

Danke sch=F6n!

-Michael

Reply to
Michael

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