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CAN Heartbeat

Anthony Jocius
Prodigy 20 points

I have several PCBA's which do not have a heartbeat.  I'm a test engineer so I'm neither the SW or HW designer.  The CANH and CANL lines have 2.4Vdc.  We use two 120 Ohm terminating resistors at each end with the inside leads tide to ground via a cap.  We use a TMS470R1B512PGET microP to our transceiver SN65HVD1050. Resistances look fine, voltages (at least on a DMM) look fine.  Not sure where else to look.  Can you recommend some other avenues which I might be able to travel?   

  • 0
    TI__Guru 62910 points
    Hi Anthony,

    If you have access to an oscilloscope it would likely help a great deal. You could then trace the signal path through from the TXD inputs to the CAN bus and out the RXD outputs to see where in the chain issues pop up. If that isn't possible, I would look into a few other things:

    - You mentioned two 120-Ohm resistors used at each end for termination; did you mean two 60-Ohm resistors giving 120 Ohms in total at each end)? That would be the typical set-up for CAN buses.

    - What voltages do you measure at TXD and RXD with your DMM? These signals are switching between 0 V and 5 V, but should spend more time high than low. The DC value I would expect, then, would be something close to 5 V. If they are stuck low then there could be a hardware issue (e.g., short-circuit).

    - Have you tried replacing the CAN transceiver with a new one (of the same part number)? This would at least let you know if something happened to damage the device.

    Regards,
    Max
  • 0 in reply to Max Robertson
    Prodigy 20 points

    Max,

    Yes you are correct regarding the resistors.  Using a DMM set to Vdc I read approximately 2.4Vdc between CANH and Ground, 2.4Vdc between CANL and Ground.  We are going to replace the transceiver and give it a go.  How critical are the resistor matches?  We are using 62 ohm resistors at 1%  tolerances.

  • 0 in reply to Anthony Jocius
    TI__Guru 62910 points
    Anthony,

    The resistance matching is not critical, so you shouldn't need to change from what you have. In addition to CANH and CANL, it would be good to check the TXD (pin 1) and RXD (pin 4) voltages to make sure that neither is stuck low.

    Max