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TCAN1044-Q1: TCAN1044-Q1 terminal resistor

Elec Cheng
Expert 7055 points
Part Number: TCAN1044-Q1


 Hi Expert,

I have a question about the terminal resistor setting. From TCAN1044-Q1 datasheet, only the first one and last one need terminal resistor as 120Ω, the rest needn't. But customer says, NXP suggest adding slightly smaller resistor for the rest CAN TRX, as shown below. That makes me confused. Could you please explain?

Thanks 

Elec Cheng

  • 0
    TI__Guru 74322 points

    Elec,

    It is true that the only required termination for the CAN bus are the 120 ohm resistors at both ends. However, if there is considerable conductor length between the middle nodes and the CAN bus, it is recommended to add a high-valued resistor to act as termination. If the conductor from the CAN bus to the node is considerably long, than this may introduce an impedance mismatch that can cause reflections, just as if there were the incorrect termination resistance in the first place.

    When adding the middle node termination, the resistance needs to be high valued so that the parallel combination of resistances doesn't bring the overall CAN bus resistance below 50 ohms.

    Please let me know if you have any other questions.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Expert 7055 points

    Hi Eric,

    Thanks for your reply. For middle node, the resistor should be placed close to CAN TRX, is it right?

    Thanks 

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Guru 74322 points

    Elec,

    Yes, that is correct. And again, you want to choose the value of the resistor(s) so the overall parallel combination of termination resistances doesn't go below 50 ohms, and preferably not even below 55 ohms. Ideally the CAN bus should be at 60 ohms, but CAN bus drivers on TI devices can work down to 50 ohms, and 45 ohms in some cases.

    Regards,

    Eric Hackett

  • 0 in reply to Eric Hackett
    TI__Expert 7055 points

    Hi Eric,

    So the overall principle is total resistor is higher than 50Ω. But for each node resistor, do we have a rule? for example, the first one and last one are larger than middle nodes.  Or more specifically, we have five mode on the CAN bus, how to design the resistors?

    Thanks,

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Guru 53575 points

    Hi Elec,

    Only the two end-points should be fully terminated with 120-ohm resistors. The actual value of intermediate nodes is not too impactful. The main concern is that partial termination is present to help dampen reflections and that the total load does not exceed the CAN driver's capabilities (~50-ohms). 

    For an example with a 5 node network (nodes A - E), nodes A and E will have 120-ohm termination. Nodes B, C, and D may have 4.7k-ohm termination to help dampen reflections. The total bus load for such a network will be around 57.8ohms. If reflections are still prominent in the system, smaller partial termination values may be used while still respecting the 50-ohm capability of the CAN drivers. If the differential voltage begins to fall too much due to the load, larger partial termination values may be used. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 7055 points

    Hi Eric,

    I have some questions,

    1. at two end-point, is 120Ω fixed? Could it be other values? 

    2. Our customer uses, 180,720,720,720,180 as NXP recommended, the overall load is 65.45Ω. Is there any problem?

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Guru 53575 points

    Hi Elec,

    This configuration works as well. The 120-ohm value comes from typical characteristic impedance of cabling used for CAN systems. Matching this impedance with termination resistance at network endpoints is the most efficient way to reduce signal reflections. Un-matched impedances will introduce reflections that are proportional to the mismatch, being most dramatic at unterminated nodes. When balancing which nodes have full or partial termination, the decision will depend heavily on system characteristics such as distance between nodes, number of nodes, and the system's tolerance to reflections. 

    If the current network configuration works with NXP transceivers, I don't see any reason to change these values when swapping to a different manufacturer. If the current system is having trouble with signal reflections, it may be worth looking into modifying the current setup to improve performance. 

    Regards,
    Eric Schott