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SN65HVD73: External resistor

Michiaki Tanii
Expert 4550 points
Part Number: SN65HVD73

Hello team,

 

There is the difference between fig37 and fig41 for placing termination resistor.

I believe SN65HVD73 should be placed termination resistor at both Tx and RX. Is my understanding correct ?

And termination resistor should be placed closer to device as possible as they can but it should be placed in front of series resistor of ⑦ as datasheet ?

 

Is the pull-up resistor at R pin mandator ?

Customer believe it is not needed if SN65HVD73 has failsafe function.

 

Is the series resistor mandator when if RE pin and DE pin tied to VCC or GND directly.

In the fig41, the resistors limit noise current at transient situation but customer would like to make sure if it is mandatory or not.

 

Thank you and best regards,

Michiaki

  • 0
    TI__Mastermind 25705 points

    Tanii-san,

    These are good questions.

    I agree it's better to include termination resistors in figure 37 as well, while the demonstration mainly focus on the ESD protection. For termination placement, I think the reasoning of figure 41 is that termination matches the wire better this way (after the series resistor, the impedance looking into the cable might change).

    The pull up resistor at R pin is not mandatory. However many people like to do that to keep the R output high while RX is driving it.

    Similarly the pull up/down resistor at DE/REB pins are not mandatory. Sometime it's concerned to directly connect a pin to the supply. The resistors can limit the current in the fault conditions.

    Please let me know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Expert 4550 points

    Hello Hao,

     

    As to termination resistor, can customer place 100 ohm resistor for termination ?

     

    Regarding the place of termination resistor, the affect by the impedance from cable would be larger than the affect by PCB pattern.

    Is my understanding correct ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru 50745 points

    Hi Michiaki,

    The value of the termination resistor should ideally match the characteristic impedance of the cable being used in the system. A common impedance value for such cabling is 120-ohms so this is the typical value we recommend for termination. While a mismatch of 20-ohms would not be ideal, it will likely only have a small negative impact and is certainly preferable to no termination. 

    Similar to matching the impedance of the cable to the termination value, it is preferable to have the PCB traces be sized and routed to have the same characteristic impedance as the cable. This would allow the signal to transfer from the cable to the board without encountering a significant change in impedance which would cause signal reflections. If board and cable impedances are set to match, the termination should be placed nearest to the transceiver to reduce the unterminated distance between the resistor and the transceiver. 

    Regards,
    Eric Schott