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ISO7241A cannot transmit signal correctly

John11513
Expert 5360 points
Other Parts Discussed in Thread: ISO7241A, SN65HVD82, ISO7341C

Hi,

I observed an abnormal behavior on ISO7241A, the status of reverse channel output (OUTD) is different from input in some condition. Below is the test circuit: (the GND1 and GND2 was connected together for convenience of test of INPUT/OUTPUT signal)

 

Here is the IND and OUTD signal waveform at power up: ( yellow waveform at channel 1 is IND, blue at channel 2 is OUTD)

You can find at the beginning the OUTD follows the IND status, while 10ms later the OUTD dropped to low and does not follow the IND anymore. This condition continued until IND is pulled low.

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I wondered what is the root cause for this abnormal condition? Is it related to some improper design or it is due to ISO7241A itself?

Thank you,

John

  • 0
    TI__Intellectual 2900 points
    Hi John,

    This is not expected behavior from ISO7241.
    What are the values of decaps being used in the design? Capacitors close to ISO7241?
    I do not see anything wrong with the schematic as such. Is it possible to share complete schematic or layout files with us for more clarity?

    Regards,
    Anand Reghunathan
  • 0 in reply to Anand Reghunathan
    TI__Expert 5360 points
    Hi Anand,

    The decaps are two 0.1uF very closed to the two VCC pins at the same side though it is not shown in schematic. I will send more detailed shcematic and pcb layout latter.

    Thank you,
    John
  • 0 in reply to John11513
    TI__Intellectual 2900 points

    Hi John,

    Thanks for your reply.

    A few questions to understand the problem better.

    1. Are you observing this behavior on multiple parts? Can you try a fresh device and see if this is repeatable?
    2. Will it be possible to share VCC1 and VCC2 waveforms alongside the IND and OUTD waveforms?
    3. Were you able to observe the ICC currents drawn at VCC1 and VCC2 when the device is in error state? Is it higher than normal current consumption of the IC
    4. Can you please provide more clarity on why the step in voltage from 4V to 5V is happening on IND after 10ms?
    5. Can you please provide a zoomed in waveform capture when the step is happening from 4V to 5V? This is see if some noise is coupling to the input pins at that point.

    Thanks & Regards,
    Anand Reghunathan

  • 0 in reply to Anand Reghunathan
    TI__Expert 5360 points

    Hi Anand,

    1. Yes, it can be observed on all parts as customer said.

    2. See below picture for IND, OUTD, VCC1 (3.3V), and VCC2 (5V):

    and in more detail during start-up:

    3. It is difficult to monitor VCC1/VCC2 current. While the temperature of the IC is cool, no matter in correct or wrong status. Are you considering whether the OUTD is pulled down by external ckt? I have checked this point. The load on OUTD is only an input pin of 74LVC07, in fact this logic gate IC has been removed when I done the test. And I did check the impedance from OUTD to GND during power down, it is hi-impedance.

    4. It is due to the 'R' output of RS485 device SN65HVD82 which is connected to IND. See below picture about IND-OUTD-OUTB waveform, right after power on, the OUTB of ISO7241A is in high state (which is consistent with INB input), this results 'R' output of SN65HVD82 is in high impedance. Such the IND is pulled to 4V by ISO7241A itself, until 10ms latter, the MCU pulls INB low and the 'R' output is actively pulled to 5V. So we can observe the IND step from 4V to 5V at this point. It seems this issue is indeed related to this 4V-to-5V step. Since the abnormal behavior disappeared once the HVD82 was removed. 

    5. Yes, i observed a negative glitch once I zoomed in this step. There is a 76ns negative glitch at this point. This should be the root cause. It is generated by HVD82 during HVD82 switched from TX to RX mode.

    Thank you

    John

  • 0 in reply to John11513
    TI__Intellectual 2900 points
    Hi John,

    Let us discuss and close this over email.
    Please drop me an email.

    Regards,
    Anand Reghunathan
  • 0 in reply to Anand Reghunathan
    TI__Expert 5360 points

    Hi Anand,

    Yes, this issue has been closed by replacing with ISO7341C.

    Thank you for help！

    John