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THVD1520: Termination resister selection.

Nam, Dino
Expert 6445 points
Part Number: THVD1520

Tool/software:

Hello team, 

At datasheet of THVD1520, we can see below comment

"To eliminate line reflections, each cable end is terminated with a termination resistor, RT, whose value matches the characteristic impedance, Z0 , of the cable. This method, known as parallel termination, allows for higher data rates over longer cable length." 

- Customer status 

1. They use a total of 32 modules. 

2. The distance from the first node to the last node is about  below 1 meter.

3. The customer have to use same PCB for all nodes. They don't know which module will be the first or last module. 

Q1 If the customer have to use same PCB for 32 node, Should we add a termination resistor to all nodes, or should we delete a termination resistor? which one is better? 

Q2 If we should add a termination resister to all 32 node, How can we fix the value? 

Please let me know your opinion. 

Thank you.

  • +1
    Guru 317210 points

    There must be exactly two termination resistors, at the two ends of the bus.

    If you cannot integrate a switched termination on the PCBs, then you must add separate termination to the bus.

    How are the PCBs connected? If they are plugged into a backplane, then you can add the termination to the backplane.

  • +1
    TI__Guru 73541 points

    Hi Nam, Dino,

    You should ideally terminate from the two furthest nodes. 

    Are you saying the longest distance between two nodes in your system is less than 1 M? What is your max data rate you're trying to support?

    You shouldn't try to terminate all 32 nodes with 120 ohm resistors. This would kill the VoD in your system, if the system bus is very short (like what you seem to suggest at 1M) then I would probably choose no termination as opposed to termination on all nodes. (Note adding super large resistors for termination to try to target 60 ohms equivalent also defeats the purpose of termination so you should not do that either.)

    As Clemen's suggested if you have some back plane that you can assign a termination resistor (on 2 nodes) then that would be ideal.

    -Bobby