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Is fail safe circuitry at the LVDS receiver a must?

Darshan Devendrappa
Prodigy 60 points
Other Parts Discussed in Thread: SN65MLVD3

Hi All,

Is fail safe circuitry at the LVDS receiver a must? If needed, how do I design a fail-safe in a multi-drop LVDS receive topology?

Your inputs are much appreciated.

Thanks,

D

  • 0
    TI__Mastermind 20820 points

    Hi Darshan,

    Failsafe circuitry is not a must and is rather a system level requirement.

    If you do use failsafe biasing you would place the network only at one end of the bus pulling the bus to the failsafe level. The other end of the bus would be terminated regularly. Here is an app note on failsafe biasing RS-485 networks that will provide you some insight as the same design rules would apply with different voltage levels:
    www.ti.com/.../snla031.pdf

    Instead of implementing failsafe biasing externally though you could use type 2 MLVDS device (receiver) like the SN65MLVD3 that has failsafe biasing implemented internally. We have a bunch of different topologies of type 1 and type 2 devices so spend some time looking through the portfolio looking for the part that best meets your design needs.

    Please let me know if you have anymore questions?

  • 0 in reply to Michael Peffers
    Prodigy 60 points
    Thanks Michael for writing back!

    I am planning to replace mlvd2 with lvds4, the reason being to eliminate additional buffer to down convert the signal to 1.8V in the case of mlvd2.

    The mlvd2 data sheet does not talk about failsafe circuitry and we did not use in our earlier designs - it worked good. Current, the lvds4 datasheet talks about fail-safe circuitry, and will include fail safe circuitry, your thoughts on replacing mlvd2 with lvds4 is appreciated. Please let me know. I'll go through the fail safe biasing of rs-485.

    Thanks,
    D
  • 0 in reply to Darshan Devendrappa
    TI__Mastermind 20820 points
    Hi Darshan,

    Like Fransisco pointed out in your other post on this topic there is no issue with replacing the MLVD2 receiver with the LVDS4 receiver. They are different devices though and not all the parameters are a 1 for 1. If the LVDS4 works in your system and allows you to lose a buffer device saving you money on the BOM then it is a good decision.

    Failsafe biasing circuits are most commonly used in noisy environments. What the failsafe circuitry does is move the VIT+ and VIT- points away from 0V to improve on the devices noise margin. If your design is not in an industrial noisy environment this extra circuitry may not be necessary. What you could do though is place the components in the layout and DNI them this way if you need to add them down the road you can.

    I hope this helps.