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TCAN1042-Q1: Can Tx/Rx could connected together

Kay Chen53
Expert 2225 points
Part Number: TCAN1042-Q1

Hi there:

I'd like to check an application for customer about TCAN1042, whether it could connect the Tx/Rx together as following picture shown?

Thanks again. 

  • 0
    TI__Intellectual 2090 points

    Hi Kay,

    The CAN protocol only allows one transceiver at a time to take priority of the CAN bus.  This means that if a transceiver transmits a dominant bit (TXD = 0), then the other transceivers on the bus will have their recessive (TXD = 1) data overwritten by the dominant bit.  

    This means that if you connect RXD to TXD, then likely the transceiver will always have TXD = 0 once a dominant bit is received on the bus.  

    Bus => Dominant,  RXD => 0,  TXD =>0,  Bus => Dominant, etc.

    This will eventually lead to a dominant-time-out error (DTO).  The transceiver will switch off its driver and enter a protected fault state.

    It is not recommended that you connect TXD to RXD for any functional CAN protocol application.  Could you let me know why you are interested in making this connection for your setup?  Are you trying to create a CAN repeater circuit of some sort, or are you interested in a different application?

    Best Regards,

    Max Megee

    Transceiver Applications

  • 0 in reply to Max Megee
    Intellectual 460 points

    Hi Max

    Correct my diagram.

    TX/RX connect to CPU and should be spilt.

    CAN bus side can be connect like I2C is righ way .

    I need 120 ohm on can bus side?

  • 0 in reply to user4786735
    TI__Intellectual 2090 points

    You are correct, the TX and RX lines should be split coming from the MCU.

    About the 120 termination resistor, will there be several transceivers connected to the CAN bus?  Or is this simply a two-node, point-to-point application?  If there are only two nodes, then you should terminate each end of the bus with a 120-ohm resistor close to the transceiver pins.  If there are several nodes, then typically you should choose the two nodes that are farthest away from each other in the network to place the two 120-ohm resistors.  I'd be glad to take a look at your network diagram if you have more than two nodes just to make a clear recommendation.

    Does this make sense?

    Thanks,

    Max 

  • 0 in reply to Max Megee
    Intellectual 460 points

    same time just have one connection on CAN BUS.

    one CAN ODB connect to OBD-II cable. other connect to cam module.

    So I just use one Tcan1024 is ok?

     

  • 0 in reply to user4786735
    TI__Intellectual 2090 points

    That should be fine to connect multiple modules to the CAN bus.  Does the CAM module and the ODB-II module have transceivers on their ends in order to receive the bus signals?  Will you be using CAN protocol from your MCU?  If so, are the CAM module and the ODB-II module capable of transmitting on the bus in order to send an acknowledgement of CAN data?

    For the 120-ohm termination resistors, I would recommend that you place one resistor close to each end-point node in the network chain.  From your diagram, it looks like the CAM module and the ODB-II module would be the best fit for the termination resistors.  In that case, the TCAN1042 device will not need termination resistance in this case.

    Best Regards,

    Max