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THVD1426: Asking for the VA and VB external resistors required for auto-direction mode

SHH
Mastermind 30886 points
Part Number: THVD1426
Other Parts Discussed in Thread: THVD1406

Hi Team,

Customer would like to design THBD1426.

are  external resistors required on VA and VB for auto-direction mode?

can customer not place 120ohm resistor between A and B?

thanks for help!

BR,

SHH

  • 0
    Guru 271170 points

    The auto-direction mode is based on a timer, and this device has fail-safe inputs. External fail-safe resistors are not needed.

    The termination resistor is not needed for the receiver itself, but if you do not have termination that matches the characteristic impedance of the line, reflections will greatly reduce the signal quality.

  • 0 in reply to Clemens Ladisch
    TI__Guru 68492 points

    Thanks for the help Clemens!

    SHH,

    I've notified a TI expert to take a look at this as well. Thanks for your patience.

    Regards,

    Eric Hackett

  • 0
    TI__Mastermind 46427 points

    Hi SHH,

    I don't see any issues - I am basically going to echo Clemens here.

    1. The pull-up / pull-down resistors are not needed for the auto-direction since it is based on a timer. U7589 has left the ability to control the shut-down pin while the U8964 will never be in shut down mode - if this is intentional then there is no issue. The device has internal fail-safe thresholds - but if you want a large margin for bus idle states they can be added but are not necessary.

    2. Termination resistor isn't necessary but as Clemens mentioned the reflections caused by the unterminated bus will degrade signal quality. However in shorter bus systems (which is going to be relative to data rate) reflections are not of a concern and the lack of termination isn't that big of a deal at 12Mbps (1426's rating) you should be okay in systems only a few meters in length but the longer you go the more concern that could be - so it is generally best practice to terminate when possible. 

    3. The only other issue is that the schematic has the THVD1406 not the THVD1426 - if this is just a typo its not much of a concern; however if it is the THVD1406 and not the THVD1426 the only real difference is that the 1406 is much slower (500kbps) than the 1426(12Mbps) and I just wanted to point that out.

    If you have any other questions please let me know!

    Best,

    Parker Dodson