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Trying to send CAN data over optical link?

Chris Schur
Prodigy 20 points

Greetings.

One of our products would benefit if we can use the existing can bus to communicate over a 20mm airgap with a pair of LED and photo diodes.  We have tried to use a Standard CAN transciever to link to the buss, then we took the TX and RX on the logic side, sent the stream over the photo link, and back onto the logic side TX and RX on another CAN tranciever, then the buss continued to the remote device.  But breaking the buss this way is not working, the communication just stops and wont work.  We tried just connecting the two CAN trancievers together on the logic sides of both > RX to TX and TX to RX and the remote device wont work either.  Trying to break the buss into two parts seems to only lock it up.  How is this done with simple Can trancievers?

Thanks!

  • 0
    TI__Guru 62920 points

    Hi Chris,

    The problem is that the CAN bus is bidirectional, and whenever a transceiver drives a dominant (logic low) on TXD it also receives a dominant on RXD.  So, if you connect the TXD/RXD lines of two transceivers, any time there is a dominant state it will totally lock up the bus (the dominant state is looped back infinitely from both sides).

    It's possible to use some logic circuitry on the TXD/RXD connections to avoid this problem, though, by controlling them properly based on the detected direction of data flow.  You can find an example implementation here:

    http://www.ti.com/tool/TIDA-01487

    This shows a case where galvanic isolation is introduced as well, but for non-isolated applications the circuitry can be simplified to use a single power rail for both sides and to bypass the digital isolator circuitry.  (Or, in your case, I believe the optical link would effectively replace the isolator shown in this design.)

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    Prodigy 20 points

    Max, thank you for your suggestions!  We will try some of your ideas this week and let you know if it worked.  We are a small sub company of the giant Nidec that develops controls, sensors and other products for the industrial robotics industry, here we are trying to develop a method of communication between factory robot and its charger base through the charging contact assembly.  Ill update here as we get this up and running...