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ISOW1412: RS485 resistors clarification

Arturo Esteban Vazquez Gutierrez
Prodigy 200 points
Part Number: ISOW1412
Other Parts Discussed in Thread: ISO1410

Hello everyone,

 

Our customer is using the ISOW1412 RS-485/RS-422 transceiver. On their PCB they placed three 49.9K ohm resistors (R221, R22, R223) and one selectable 121-ohm resistor (R224) as shown the attached f1 image.

On the ISOW1412 datasheet we didn’t found information about the previously mentioned resistors, as per our understanding R221 and R223 are needed by the typical RS485 transceivers and when consulting the documentation of the ISO1410 (which is a similar part number), we noted that such IC does not need those as shown the attached f2 image.

Could you please confirm if the ISOW1412 RS-485/RS-422 transceiver needs the R221, R22, R223 resistors that our customer placed?

 /cfs-file/__key/communityserver-discussions-components-files/1013/images.zip

Any guidance will be appreciated,

Best regards,

Esteban V.

  • +1
    TI__Guru 54155 points

    Hi Esteban,

    Thanks for reaching out and for your interest in ISOW1412.

    Most present day RS-485 transceivers have internal failsafe biasing resistors and do not require any external biasing resistors, and so is the case with ISOW1412 as well. Hence, you do not have to use R221, R222 and R223. But if you do want to use, make sure overall resistance value between the RS-485 bus terminals (A and B) matches the termination resistance of the cable used which in most cases is going to be 120Ω.

    You can refer the below document for information related to calculating the failsafe biasing resistor values for Rs-485 bus. Thanks.

    RS-485: Passive failsafe for an idle bus


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 200 points

    Hello,

     

    Thank you for the quick response, and the clarification.

     

    Best regards,

    Esteban V.