questions regarding CAN transceivers

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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 (
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 (
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.



Re: questions regarding CAN transceivers

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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.

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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 (=<125 Mbps). The key is the steepness of the slopes which
may cause EMI. DOn't make your bus faster than necessary.

Mit freundlichen Grüßen

Frank-Christian Krügel

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

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Re: questions regarding CAN transceivers
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Well - how can these parts be marketed if they aren't functional?
Furthermore - they were working perfectly when it was entirely a 3.3V

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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 schF6%n!


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