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Original question:

AM26LV32E: About Fail-Safe

AM26LV32E-EP: Terminated Fail-Safe status

Brian Gray
Prodigy 30 points
Part Number: AM26LV32E-EP
Other Parts Discussed in Thread: AM26LV32E, AM26LV32

According to the latest AM26LV32E data sheet (Revision D), the unit does not support Shorted or Terminated Fail-Safe conditions.  The latest Revision of the AM26LV32E-EP data sheet (Rev A on TI website) states that it does still support Shorted or Terminated Fail-Safe conditions.  Can someone verify for me the expected behavior of the AM26LV32E-EP with a termination resistor installed when its inputs are tri-stated?

Thanks,

Brian

  • 0
    TI__Mastermind 43205 points

    Hi Brian,

    The "EP" version must have not been updated when the non-"EP" version of the datasheet was. Thanks for bringing this to our attention. 

    Both versions of the device should be open fail safe only as stated in the non-EP version datasheet. Voltages in-between -200mV and 200mV could cause random output of 1 or 0 from the receiver output pins. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 30 points

    Are there any extended temperature versions of the AM26LV32 part number?  Or is the AM26LV32I (-40 to +85C) the widest temp range for a chip that has the Terminated Fail-Safe option?

  • 0 in reply to Brian Gray
    TI__Mastermind 43205 points

    The -40C to 85C is the highest temp rated version of the AM26LV32. 

    Do you only have a 3.3V supply available for the application? It seems there may be some similar devices that have higher temperature range with a different supply voltage (5V) - are there are design needs for a part? 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 30 points

    We need something that uses 3.3V logic at the output.  5V outputs are not an option.

    Can you clarify for me (to make sure I understand) the phrase "Terminated Fail-Safe"?  Am I interpreting it correctly by saying with our Termination Resistor between the + and - inputs, if the inputs are Tri-State/High Impedance, the Terminated Fail-Safe would result in a Logic 1/High state at the chip's output?

  • 0 in reply to Brian Gray
    Guru 243040 points

    Yes; "terminated fail-safe" means that the termination resistor shorts the inputs together so that the differential input voltage is zero, and that the receiver is guaranteed to output 1 for this.

    The easiest way to solve this might be to add fail-safe resistors (pull-up and -down) to make the voltage drop over the termination resistor larger than 200 mV.

  • +1 in reply to Brian Gray
    TI__Mastermind 43205 points

    Hi Brian,

    For this device terminated fail safe is when the inputs are opened the receiver  will output high. Idling (terminated) or short conditions will not be protected against and "R" state is undefined

    To add idling failsafe protection you can add failsafe biasing resistors - explained here: https://www.ti.com/lit/an/slyt324/slyt324.pdf  - but a short between A and B will create a differential voltage of 0V so the receiver output will be undefined.

    Legacy devices like this generally have this disadvantage. Modern devices have both thresholds below 0V to help mitigate these issues. However we don't have a good replacement with this feature as this part - along with other parts in a similar configuration  - are legacy devices and generally aren't protected in this case.

    Best,

    Parker Dodson