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Original question:

AM26LV32E: failsafe output logic

AM26LV32E: Output logic under "Fail-safe", but with termination resistor

Chen Jia
Intellectual 850 points
Part Number: AM26LV32E
Other Parts Discussed in Thread: AM26LV32

Tool/software:

To whom it may concern:

We have a circuit with this IC as a differential signal command receiver, and we would like to keep the output high when input open (G always high). Based on the IC spec, it should have "Fail-safe" feature, and the output should be high when open:

"Removing the uncertainty of random output states, modern transceiver designs include internal biasing circuits that put the receiver output into a defined state (typically high) in the absence of a valid input signal. "

We build two rounds of samples with this AM26LV32E IC, and the first round was all good, but the second round with different batch of ICs showed different result. All these ICs were purchased from the same qualified distributor. Could you help to confirm:

If input is "OPEN", but with 100ohm termination resistors, the output should be high? Termination resistor is normally required for RS422 design.

Thank you very much!

Chen

  • 0
    Guru 316380 points

    Section 8.3.2 lists three conditions where a fail-safe would be useful (open/short/idle), but then mentions that the AM26LV32E's fail-safe works only for open inputs.

    In this circuit, the inputs are not open. The termination resistor forces VID to 0 V, so the output is undefined.

    The AM26LV32 without E uses different logic and has a defined output state in all three fail-safe conditions.

  • 0
    TI__Guru 71910 points

    Hi Chen,

    Clemens is correct as usual.

    These devices look like they have a weak internal pull up on A and pull down on B which provide a known state during open conditions w/o the termination present. 

    If you need to ensure a known state with a termination resistor present, we generally tell customers to place an external fail safe biasing circuit if the device does not already have an internal fail safe (generally you can tell by looking at Vth+ and seeing if it is negative or below GND). 

    -Bobby