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AM26C32: Fail-safe of AM26C32

Tank.Xue
Prodigy 90 points
Part Number: AM26C32
Other Parts Discussed in Thread: AM26LV32, AM26LV32E

Hi TI experts,

I am using the differential line receiver AM26C32 and I found a quesion:

when the input of AM26C32 is open, its output is not all high, which is not consistent with the fail-safe function mentioned in the datasheet.

Here is my schematic, please help me check the problem, thanks!

Best Regard!

Tank.Xue

  • 0
    Guru 296650 points

    The schematic does not show any open inputs. When the inputs are grounded or connected with a termination resistor, they are not open.

    You should leave the 4A/4B pins unconnected, or connect them to different voltages.

  • 0
    TI__Mastermind 48267 points

    Hi Tank,

    Is the bus connected to a driver and driver is just high impedance?

    If that is the case it may not be an open bus but an Idle one. This device is a bit tricky when it comes to the failsafe because its not great - the VIT+ and VIT- thresholds are not failsafe protected (which is how it is usually done on modern devices so that IDLE/OPEN/ and SHORT conditions are failsafe not just OPEN).

    Using passive failsafe resistors on the bus (you should need 2 resistors per differential bus - A to VCC and B to GND) - I have provided an app note on how to size these resistors. They will keep the bus at a high output for OPEN and IDLE conditions  https://www.ti.com/lit/an/slyt324/slyt324.pdf. This method is still a common use case even on more robust input failsafe schemed devices.

    Best,

    Parker Dodson

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Hi Ladisch

    Thanks for the information you provided, so that I understand the use of fail-safe!

    Please recommend other chips for us: even with termination resistors, the fail-safe function can be effective, Thank you very much!

  • 0 in reply to Parker Dodson
    Prodigy 90 points

    Hi 

    Thanks for the information you provided, so that I understand the use of fail-safe!

    Please recommend other chips for us: even with termination resistors, the fail-safe function can be effective, Thank you very much!

  • +1 in reply to Tank.Xue
    Guru 296650 points

    The only four-channel receiver with this feature is the AM26LV32 (but not the AM26LV32E), which runs at 3.3 V.

  • +1 in reply to Clemens Ladisch
    Prodigy 90 points

    Thank you very much.