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AM26LV32E: Failsafe function of RS485 line receiver

junya.miyamatsu
Prodigy 10 points
Part Number: AM26LV32E
Other Parts Discussed in Thread: AM26LV32, THVD1450

Dear Experts

While checking the operation of a circuit using AM26LV32EIPW and AM26LV31EIPW, the following events occurred.
I would like to ask the following regarding the fail-safe function.

[circuit]
・Master and Slave are 1:1 RS485 half-duplex communication (communication speed: 20MHz)
・Both Master and Slave use AM26LV32EIPW and AM26LV31EIPW, and they are all connected to the same bus.
・Each differential line is equipped with a 100Ω terminating resistor (there is no external pull resistor for each differential line. expecting internal fail-safe behavior)

[phenomenon]
・During the period from when the Master completes sending the frame to when the Slave sends the frame (*1), the differential input of the receiver is around 0V, and during this time the output of the line receiver momentarily changes from High to Low. Sometimes. The slave will mistakenly detect that it is a frame from the master. Occurs when the cable length is long.
*1: At this time, both Master and Slave negate "G" of the line driver, resulting in Hi-Z.

[What you would like to ask]
According to P7 of your document "TIA/EIA-485 (RS-485) Interface Circuit" (JAJA179), if you use a transmission termination resistor (about 100Ω), the differential voltage will be around 0V and the receiver state will be no longer switch
My understanding is that the internal fail-safe does not work well, but does the description in the above document apply to the AM26LV32E as well?

Best Regards,

Miyamatsu

  • +1
    Guru 316690 points

    The AM26LV32E has a fail-safe function for open inputs. But open inputs happen only if the termination resistor is disconnected, so it is not useful for your circuit.

    The AM26LV32 (without E) has fail-safe also for shorted inputs, but much lower ESD protection.

    You can

    • add fail-safe resistors to your bus lines; or
    • use the AM26LV32, and add external ESD protection; or
    • use modern transceivers like the THVD1450 (which all have exactly one driver and one receiver).
  • +1
    TI__Mastermind 40076 points

    Hi Miyamatsu,

    Clemens' is correct here- the failsafe function for this device only covers open circuited bus (not during idle or short circuited bus is covered in more modern transceivers).

    To resolve the device in idle and open conditions, they can add external fail-safe protection by using external biasing resistors (~680Ohms) to one node. Here is an app note that explains this implementation.

    Its important to note that if short protection is also required or the external biasing resistors implementation are not feasible, you will need to use modern transceivers (such as the ones Clemens' recommended). 

    Regards,

    Jack  

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points
    Hello Clemens

    Thank you for your reply, including the detailed solutions.

    I understand that the above circuit does not open, so the failsafe circuit does not work.

    The information I provided was a little lacking, the phenomenon described in [Phenomenon] occurred only when the boards with the above circuits communicate with each other. Our current design is a circuit that communicates with a master that has an external fail-safe resistor, so I understand that there is no problem.

    I will also check modern tranceiver and use it as a reference for future designs.


    Best Regards
    Miyamatsu
  • 0 in reply to Jack Guan
    Prodigy 10 points
    Hello Jack

    Thank you for your reply and the link to the reference document.
    I will check the contents here.

    There was a little lack of information in my description. The phenomenon described in [Phenomenon] occurred when the boards equipped with the above circuit communicated with each other as a test.
    I understand that there is no problem because it is designed to be used for communication with a master that has an external fail-safe resistor.

    In future designs, if short protection is required and an external fail-safe resistor cannot be implemented, we will consider adopting modern tranceiver.

    Best Regards
    Miyamatsu