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TCAN1042-Q1: CAN physical layer multiplex

Xianghao Meng
Prodigy 670 points
Part Number: TCAN1042-Q1
Other Parts Discussed in Thread: THVD1500, SN65HVD1780-Q1

Hi,

In my customer's application, there is only one UART port available on processor and the signal will be translated to CAN physical layer by TCAN1042-Q1. But there will be two different types of slavers with different protocol connected to this CAN physical layer. Because two protocol may have contentions, it is under consideration to use CAN transceivers to separate the interface physically. Do you think it is possible to use two pieces of TCAN1042-Q1 to separate the interface from processor physically? Or do you have other single product solution to achieve same function such as a multiplex for one input for Rx, Tx and two selectable  CAN output. Thank you.

Best regards,

Xianghao

  • 0
    TI__Guru 62920 points
    Hello Xianghao,

    It sounds like using two TCAN1042 transceivers along with some mixing or logic circuits on the TXD and RXD signals to the UART would accomplish what you need. If you are not tied to the CAN physical layer, you might consider RS-485 instead. RS-485 transceivers typically have inputs that can be used to disable the driver and receiver circuits. An example of such a device is THVD1500.

    By the way, if you could draw out a block diagram it may help me to make sure I understand the intended connections.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Prodigy 670 points

    Hi Max,

    Thank you for your reply, It's helpful. I didn't think about RS-485 physical layer before, because it's for automotive application. CAN is easy for customer to accept. And I am not sure if there is any RS-485 transceiver in Q grade. Below is my drawing diagram for separate CAN physical layers. Idea 1 doesn't work very well because the Rx pin will be pulled high by CAN transceiver in standby mode. Do you agree? I am looking at the solution for the idea 2. There are a lot of dual SPDT analog switch available, but most of them have large R(ON) resistance about 120ohms. Do you think 120ohm R(ON) for analog switch is good or not? Thank you.

  • 0 in reply to Xianghao Meng
    TI__Guru 62920 points
    Xianghao,

    Since CAN buses typically have an effective resistance of ~60 Ohms (due to use of two 120-Ohm termination resistances in parallel), having a series resistance of 120 Ohms will cause too much signal attenuation from the CAN driver. I'd recommend targeting a maximum series resistance of 5-10 Ohms to avoid this problem. Another challenge with SPDT switch selection is that most automotive applications would require the CAN bus to be able to operate over the full +/- 12-V common mode voltage range. Because of this, you may find it is easier to use two transceivers and place the switches on the TXD/RXD lines as well.

    In your Idea 1, you are right that one problem would be contention between the two RXD outputs. This could be solved by connecting each RXD output to separate inputs of an AND gate whose output connects to the MCU. This would mean that if either RXD is low (dominant), then the MCU would see a low level input. This could work, then, as long as the "unused" CAN bus does not trigger the RXD line of its transceiver to go low.

    It sounds like CAN is the better choice based on your comments, but I did want to mention that we do have some Q100 RS-485 transceivers as well - see SN65HVD1780-Q1.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Prodigy 670 points

    Hi Max,

    Thank you. It solved my issues.

    Best regards,

    Xianghao