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SN65LVDT33: Outputs Guaranteed

Fred Brehm1
Prodigy 20 points

Part Number: SN65LVDT33
Other Parts Discussed in Thread: SN65LVDS33

Hi, Your SN65LVDT33 data sheet reads that the outputs are guaranteed to be logic HIGH when the inputs to the part are open, even though there is a 100 ohm terminating resistor in the IC between the differential inputs of the device.  Does that mean that similar devices, like an SN65LVDS33, that needs an external 100 ohm terminator, also guarantee that the outputs of the device are guaranteed to be logic HIGH, with no other pull-up or pull-down resitors and only the 100 ohm line-to-line termination resistor in circuit?  Just curious as some of us engineers here are divided over the  interpretation of the devices data sheets.  Thank you for settling this once and for all, or so I hope.

  • 0
    TI__Mastermind 29672 points

    Hi Fred,

    I've assigned this to our LVDS expert, Matt. Although he is out of office today, he should be able to get you a response tomorrow.

    In the meantime, here is our FAQ page for LVDS that may be helpful:  [FAQ] LVDS / M-LVDS Transceiver Resources 

    Best,

    Ethan

  • 0
    TI__Mastermind 28775 points

    Hi Fred,

    Both with and without the internal termination resistor you are guaranteed to get output high. The truth table in the device datasheet can help clarify this.

    When the differential inputs are open, your output will be high, regardless of 'T" or "S" variant. The active failsafe circuit used accounts for even an open input with the internal termination resistor (see Active Fail-Safe in TI's LVDS Receivers section 2.2).

    Best Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 20 points

    I guess my real question was not clearly stated.  Given a non "T" variant LVDS receiver that is on a board with the external termination resistor connected across the differential inputs of the device. The differential inputs also connect to a cable to another board where the diff driver is located.  If the cable is not connected, or the driving board is not powered, and our receiver board is powered, does the failsafe still operate to drive the single-ended output of the receiver to a solid logic high?  Or really briefly stated, does the failsafe still operate even when a line to line termination resistor is connected to the diff inputs of an LVDS receiver?

  • 0 in reply to Fred Brehm1
    TI__Mastermind 28775 points

    Hi Fred,

    Yes, in the case of a disconnected cable or powered-down transmitter, failsafe will still continue to operate and drive the receiver output to logic high. The datasheet confirms this. 

    So if the driver board is unpowered, and the receiver board is powered, failsafe on the SN65LVDT/S33 will still drive the single-ended output high. The same is true if the cable is disconnected as well. 

    Fred Brehm1 said:
    Or really briefly stated, does the failsafe still operate even when a line to line termination resistor is connected to the diff inputs of an LVDS receiver?

    Yes, it will still operate. 

    Best Regards,
    Brandon Fisher