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Original question:

TCAN1043-Q1: CANopen

TCAN1043-Q1: Internal logic in different CANH/CANL states

Rickey Li
Prodigy 70 points
Part Number: TCAN1043-Q1
Other Parts Discussed in Thread: TCAN1043

Hi TI experts,

When I use TCN1043/TCN1432, if CANH and CANL in following three states, whether the internal logic of the chip will be affected? How will the internal logic be different? Whether the chip can work normally?

1:CANH connect to power supply;

2:CANL connect to GND;

3:CANH connect to power supply while CANL connect to GND. 

Look forward to your reply!

  • 0
    TI__Guru 53515 points

    Hi Rickey,

    The TCAN1043 has fault protection on the CAN pins up to ±58V. The pins have internal current limits to protect them from overcurrent during such fault cases. Therefore there is no risk of damage to the device if the CAN pins are shorted to the supply or GND during normal operation. 

    Note that while it is possible for communication to be maintained during such shorts, the integrity of the signal will be severely decreased and may not be possible at higher data rates. 

    Let me know if you have any more questions. 

    Regards, 
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Prodigy 70 points

    Hi Eric,

    Thanks for you prompt reply.

    You mean, TCAN1403 CANH can short to power supply and CANL can short to GND without influence in communication logic, but for higher data rate the signal will be decreased.

    Is the same for TCAN1042? No influence on communication logic？

  • 0 in reply to Rickey Li
    TI__Guru 53515 points

    Hi Rickey,

    The internal logic of the device is not affected in any way by these shorts. The shorts on the CAN bus can impact the CAN signal and may make it more difficult or impossible to communicate on the network. This is a characteristic of all CAN systems and is not unique to the TCAN1043 or TCAN1042, but you may see different performance during these short cases as driver strengths may vary between different devices. 

    Regards, 
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Prodigy 70 points

    Hi Eric,

    As you said, when customer short on CAN bus, at the speed about 2M, the CAN signal be incorrect, whether there are external adjustment methods to improve the signal integrity of high-speed signal in this conditions?

  • 0 in reply to Rickey Li
    TI__Guru 53515 points

    Hi Rickey,

    When one the differential signals of the CAN bus is shorted, the system essentially turns into a single-ended communication scheme. In this case, we would want to reduce the transition time of the non-shorted line as much as possible so that the recessive signal can propagate correctly. The primary way of doing this will be to reduce the overall bus capacitance on the signal lines. Equally important will be to reduce the network complexity as much as in practical to limit the ringing of the signal across the signal lines. 

    CAN is not designed to operate consistently when one of the differential signal lines is shorted to some DC fault condition. There are possible contingencies that allow CAN to operate at lower data rates under such fault conditions, but without nominal conditions it is not possible to reach the maximum data rates offered by the interface. 

    Regards, 
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Prodigy 70 points

    Hi Eric,

    Thank you for your continued support！

    My customer wants to know whether there is a threshold for the effect of bus capacitance on speed, how to evaluate? Such as when CANL short to GND, for 2M speed, how to evaluate the maximum bus cap without affecting the transmission of single-ended communication?

    Looking forward to you reply！

    Regards, 
    Rickey Li

  • 0 in reply to Rickey Li
    TI__Guru 53515 points

    Hi Rickey,

    I think there are too many variables at play here to give a meaningful answer to this question. This is not a supported function of the CAN interface and there is not a lot of data on the complex topic of signal reliability outside to nominal conditions for CAN. I think the engineering time would be better spent finding ways to resolve a bus fault using the information provided by the TCAN1043's bus fault detection feature than trying to design a system that will work in some marginal fault conditions (keep  in mind that most fault conditions will make it entirely impossible to communicate in any capacity on the CAN bus).

    Regards, 
    Eric Schott