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SN65LVDS180: Strange LVDS behavior

Bill Adams1
Prodigy 190 points
Part Number: SN65LVDS180
Other Parts Discussed in Thread: SN65LVDM176

We are using the SN65LVDS180 device to drive a differential signal across a cable which is 50 feet long.  The same device (SN65LVDS180) is used on both ends of the cable as the driver and the receiver.  Some boards seem to work fine.  Other boards exhibit strange behavior.  One side of the differential pair does not switch properly but the other side does.  The strange part is that it seems to follow the receiver and not the driver.  Do you have any explanation for this behavior?

  • 0
    TI__Mastermind 35130 points

    Hi Bill,

    Can you provide waveforms as well as the schematic for your design? Also, when you say it follows the receiver, does this mean you've tried switching out the receivers and observed the problem when the receiver was paired with a different driver?

    Regards,

    I.K.

  • 0 in reply to Ikechukwu Anyiam
    Prodigy 190 points

    We actually have the transmit and receive pairs tied together.  We selectively control transmit or receive with the DE /RE pins.  They are also tied together.  We replaced the device on a failed board and it worked.  We need to do some more investigation to determine if the transmit or receive failed.  The same devices are on both ends of the cable.  The remote end gets its power from the local end.  Could hot-plugging result in a failure.  The TX or Rx function on the remote end is hard-wired with resistors.

  • 0 in reply to Bill Adams1
    Prodigy 190 points

    The device that was replaced was always in receive mode.  I was just wondering if one of the protection diodes on the Rx or Tx could have failed shorted causing what we see.

  • 0 in reply to Bill Adams1
    TI__Mastermind 35130 points

    Hi Bill,

    The TX/RX pairs should not be tied together. The stub created by this as well as additional loading from the TX/RX can cause signal integrity issues. If you have a half-duplex application you should use the SN65LVDM176 instead. But yes hot plugging can cause failures if the absolute max specs in the datasheet are violated. 

    Regards,

    I.K.