CAN bus optical isolation

[kender]

Colleagues,

I'm concieving a medical instrument with multiple PICs, which are talking to each-other through a CAN bus. I need to isolate one of the PICs. Is it possible to optically isolate a part of the CAN bus? As far as I understand CAN bus is differential, and doesn't have a dedicated TX and RX lines. Optical isolation, however is directional. Could anyone point me to the appnone or a schematic?

Nick

[Yashu]

[treitmey]

or perhaps opto couplers. I use fairchild HCPL-0600
http://www.fairchildsemi.com/collateral/opto/optocoupler_ssr.pdf
and for the DC-DC conversion I use the NTE0505M from C&D electronics.
this is for RS485 isolation. I am not sure if they would be the same.
I am isolating 5V signals.

[kender]

[libor]

I remembered reading an article about galvanic isolation of the I2C bus, which is in this this regard has the same challenge having a bidirectional data line. The article shows and explains how to use two optocouplers to split the bidirectional dataflow and recombine it. This might be of help to you, if you can adapt it to the diffential signal lines somehow (if all else fails maybe with two CAN transceivers, one on each side)
Here is the article.

[libor]

A follow up:
You can't spare using CAN-bus transceivers on each side of the isolation border. These transceivers have splitted unidirectional data lines on their host side anyway, so there is no need for tricking this bidirectional thing into the circuit, just use one optocoupler on each direction per side. A total of four will be needed for one link. (you may have up to two or even four of them in one package)
This is also the way industrial CAN bus products are made, look at this Application Note at agilent.com: AN 1321- High Speed CMOS Optocoupler Applications in Industrial Field Bus Networks (search for "AN 1321" on their site)

[Sandman]

Do you have to isolate the CAN-transceiver from the bus? Why?
If not: you could use ordinary optical (like opto-couplers) or digital (like; ADUM1XXX from analog div.) isolators between the PIC and the transceiver. This method is used in space application where redundancy and security is a major design factor which it also is in the medical industry.
The CAN transceivers are usually very good when it comes to stay functional in unfriendly environments.

[Yashu]

[asmallri]

[kender]

I see: the CAN-to-plain-serial chips are unavoidable. Although, CAN is becoming increasingly popular, and hopefully some day some company will make a CAN optoisolation chip, which would integrate all the necessary stuff.

Nick

[MikeW]

here is what you want.

check the circuit diagram, the PIC is opto isolated from the CAN network.


http://www.triangledigital.com/man2020f/ch7can.htm


Mike

[kender]

Thanks Mike!

I have also found this reference design, which is very similar:

http://www.slac.stanford.edu/~wtmeyer/detcon/can_optoiso.ps

If you can not open the PoscScript, I can send you the PDF[/url]

[kender]

By the way:

Is there a standard connector for CAN?
Is there a free version of a CAN bus spec?

[PCM programmer]

For CAN bus connectors, we use this document:
CiA DR 303-1 V1.3: CANopen cabling and connector pin assignment

It's available here:
http://www.can-cia.org/downloads/ciaspecifications/

[kender]

PCM programmer,

I have requested the "CiA DR 303-1 V1.3: CANopen cabling and connector pin assignment" on the CiA web site, but they didn't send it to me. Could you send this document to me?

Nick
[email protected]