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ISO1432: The fail-safe function does not work properly even when nothing is connected

Shinichi Inoue
Guru 21696 points
Part Number: ISO1432

Tool/software:

Dear Specialists,

My customer is encountering the problem regarding ISO1432.

I would be grateful if you could advise.

---

We are using a half-duplex connection between ISO1432B and a connector in our equipment.
Regardless of whether an external device is connected or not, it continues to send commands to external devices at regular intervals.

We have found that the fail-safe function does not work properly even when nothing is connected to the connector, and we are investigating the cause.

With this background in mind, I have three questions.

Q1. Will the fail-safe function not work when ISO1432B is connected in half-duplex?

Q2. There is a half-duplex product in the same series called ISO1430B.

Will the fail-safe function function more correctly with this product?

Q3. Is it advisable to attach an external resistor to ensure that the fail-safe function works properly?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • +1
    Guru 317090 points

    On a half-duplex bus, a transceiver will always read back its own transmitted data (because it is valid data, not a failure). To prevent this, pull /RE high when sending.

  • +1 in reply to Clemens Ladisch
    TI__Mastermind 29265 points

    Hello Shinichi-san

    Thanks for reaching out. The other half-duplex transceivers will operate the same way, and no external resistors are needed to operate the fail-safe feature. To stop the transmission of data while the connector is disconnected, disable /RE as Clemens suggested. 

  • 0 in reply to Andrew J
    Guru 21696 points

    Hi Clemens and Andrew,

    Thank you for your reply.

    I shared the information /RE is H when connector is disconnected with the customer.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21696 points

    Hi Clemens and Andrew,

    The customer tried your suggestion, but the situation did not change.

    Clemens Ladisch said:
    On a half-duplex bus, a transceiver will always read back its own transmitted data (because it is valid data, not a failure). To prevent this, pull /RE high when sending.

    The data sheet says that the fail-safe works, but I am having trouble explaining why it does not work properly.

    On the other hand, the fail-safe resistor works when an external resistor of 10kΩ is attached,

    So my question is, what is the bias resistance of the ISO1432 fail-safe?

    According to the ISO1432 data sheet, the output is in an undefined condition, so I believe the bias resistance is 15kΩ or more (VID=19.92V or less).

    If so, that would explain why the fail-safe does not work properly when a 120Ω termination resistor is attached.

    Regarding failsafe resistor, we have continued to use the same external resistor constant as the old product, and have not had any particular problems.

    I understand that the constant should be smaller to operate correctly as a fail-safe.

    With this external resistor, VID is about 0.03V, which satisfies the ISO1432 function table requirement of 0.02V or more, so it should not go into an undefined state.

    ｰｰｰ

    The customer must explain this to the end user.

    If you can comment by tomorrow, it would be helpful.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 317090 points

    The ISO1432 does not use internal fail-safe resistors (the Ii1 specification is the same for the A and B pins); the fail-safe function is implemented by moving the input thresholds.

    There is an error in the datasheet; the threshold is not 0.02 V but −0.02 V. With a differential input voltage of 0 V, the output is guaranteed to be high.

    You have observed that you need external resistors. What is the voltage over the termination resistor without them? Is it exactly 0 V, or below −0.02 V?

  • 0 in reply to Clemens Ladisch
    Guru 21696 points

    Hi Clemens,

    Thank you for your reply.

    The customer has an additional question.

    Could you please advise.

    ---

    Clemens Ladisch said:
    The ISO1432 does not use internal fail-safe resistors (the Ii1 specification is the same for the A and B pins); the fail-safe function is implemented by moving the input thresholds.

    According to the ISO1432 datasheet P.25 9.3.2 Failsafe Receiver,

    The internal failsafe biasing feature removes the need for the two external resistors that are typically required with traditional isolated RS-485 transceivers as shown in Figure 9-3

    Based on this statement, I think there is some kind of bias source, even if it is not a resistor. 

    So I think the answer contradicts the datasheet description.

    I am confusing which should I trust, the data sheet or the answer?

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 317090 points

    The ISO1432's failsafe biasing is not implemented with pull-up/-down resistors at the pins, but with some other mechanism. (I do not know details; section 9.4.1 does not show the A/B pins.)

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 29265 points

    Hello Shinichi, 

    Thank you for the clarification. As Clemens suggested, there is an internal biasing set-up that removes the need for external failsafe resistors. You can trust the datasheet answer. 

    Clemens Ladisch said:
    You have observed that you need external resistors. What is the voltage over the termination resistor without them? Is it exactly 0 V, or below −0.02 V?

    Please provide a response to Clemens's previous debug step. 

    Shinichi Inoue said:
    We have found that the fail-safe function does not work properly even when nothing is connected to the connector, and we are investigating the cause.

    From earlier in the thread, can you please provide details on how the incorrect behavior was identified? Please include a scope shot of the failure that captures A, B, R, and D pins as well as status DE and /RE.

    Best,
    Andrew