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AM26LV32E:Question about failsafe

Yo Oaku
Prodigy 10 points
Part Number: AM26LV32E
Other Parts Discussed in Thread: AM26LV32

Hello.

I would like to ask you two questions about the fail-safe function of the quad differential line receiver AM26LV32E.

1)  Does the fail-safe function ensures functionality in the event of a short-circuit failure?

2)  Is it possible to use the failsafe function to fix the output to High when two drivers are used in one AM26LV32E and one unplugged Cable, as shown in Figure 8-11 on page 16 of the AM26LV32 data sheet?
Also, is it possible to fix it to High even when both cables are unplugged cable?

  • 0
    Guru 316920 points

    The AM26LV32E has a much weaker fail-safe implementation than the AM26LV32. The AM26LV32E's outputs go high only if both inputs are open, i.e., when there is no termination resistor. This is not very useful in practice; you'd have to ensure that the termination resistor is removed together with the cable.

    You can add fail-safe pull-up/-down resistors to the bus. Alternatively, use modern devices (THVDxxx) that have better fail-safe implementations (but they exist only as 1+1 transceivers).

  • 0
    TI__Guru 52262 points

    Hi Yo,

    What Clemens said is correct - but just to verify your questions:

    1. No the AM26LV32E is only open circuit fail-safe protected so short circuit fail-safe is not included in this device (A to B shorts won't damage the device - but output will not be defined during short) 

    The functional block diagram shown above has the 200k resistor highlight. The A Channels are weakly pulled to VCC and the B channels are weakly pulled to ground. So when A and B are open circuited the device will default to a logic high input. However if A and B are shorted or A and B are terminated the fail-safe feature doesn't apply - so this device isn't short or idle fail-safe - just open failsafe.

    2. So no the AM26LV32E does not have as much of a robust failsafe as the AM26LV32 - the figure 8-11 shows the receiver is terminated so the AM26LV32E will not have its failsafe active in that situation. 

    You can add idle fail-safe (same as the case described in figure 8-11) by adding 2 biasing resistors - instructions on to do so are found here: https://www.ti.com/lit/an/slyt324/slyt324.pdf - please note that the termination for 422 is 100 not 120 as shown in note (note was focused on RS-485 - but from a design point of view its the same exact process - just some of the numbers change) 

    Short bus fail-safe really can only solved via an integrated short protection feature - for four channel devices the AM26LV32 is the option for short fail-safe. Otherwise we do have many modern devices in the THVD family of devices that have more robust fail-safes - but they do vary in how they work - the most robust is a fail-safe where VIT+ is less than 0V - as they aren't really timing dependent.  

    Please let me know if you have any other questions

    Best,

    Parker Dodson