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DS26C31MQML: about the failure mode

daejin kang
Intellectual 320 points
Part Number: DS26C31MQML
Other Parts Discussed in Thread: SN65HVD78,

Hi, I'm daejin

I am using the part for 422 communication.

the 422 communication test result used the part is success. 

but, unfortunately after test I found an odd problem during to impedance check.

If the part is nominal, impedance between Pin 2 and Pin 8(GND) is greater than 1 Mega ohm.

but, when I measured the impedance, the measured value is approximately 75 ohm and Pin 3 to Pin 8 is 6.8 k ohm.

Actually, 422 communication was fully succeeded but, after test impedance value was odd.

I'm wondering why this problem occurred.


could you help me about this issue?

thank you in advanced

daejin.

  • 0
    TI__Mastermind 25705 points

    Daejin,

    Can you give some detail about the impedance measurement, like the equipment and status of the device (such as pin1, pin4, pin12, pin16)? Also I'm not sure where you get 75Ohm. Did you measure between pin 2 and pin 3? I'm looking forward to your reply.

    Regards,

    Hao

  • 0 in reply to Hao L
    Intellectual 320 points

    Dear Hao,

    thank you for answer.

    I use only A of 4 channels.

    I measured the between pin 2 and pin 8, and then this was 75 ohm.

    The between pin 3 and pin 8 was 6.6 Kohm

    The pin 2 and pin 3 is approximately 6.7 Kohm

    Let me know if you need more information.

    Thank you in advanced.

    daejin.

  • 0 in reply to Hao L
    Intellectual 320 points

    additional information

    My focus on the issue is why this happens. ex, output over current(sourcing), reverse current to output pin(sinking)

    and, even if the part has the same resistance value as mentioned above (75 ohm, 6.6 Kohm ), this part is operates normally for 422 communication.

    thank you in advanced.

    daejin

  • 0 in reply to daejin kang
    TI__Mastermind 25705 points

    daejin,

    When you did the measurement, was the device powered up with a 5V supply? What's voltage on the input (pin 1)?

    FYI - you can find the driver's equivalent schematic on the other RS485/422 devices, like this one of SN65HVD78 (Figure 18 on page 16), which might be helpful.

    www.ti.com/.../sn65hvd78.pdf

    Regards,

    Hao

  • 0 in reply to Hao L
    Intellectual 320 points

    Dear Hao,

    I measured pin impedance when only this device is non operating.

    the operation voltage of DS26C31 is 5 V.

    Hao, the output port(pin2, pin3) of  DS26C31MQML is installed the ESD protection diode?

    I am wondering why the measured impedance is lower than the initial impedance (before 422 communication).

    Below is the impedance value measured before and after 422 communication.

    Additionally, 422 communication has been performed for about 3 months and while I was doing the test, the communication error has been not found.

    measured pin Initial measured value (before 422 communication) after 422 communication
    pin 2 to pin 3 Open loop 6.7 Kohm
    pin 2 to pin 8 2.2 Mohm 75 ohm
    pin 3 to pin 8 2.2 Mohm 6.6 Kohm

    Thank you in advanced.

    daejin.

  • 0 in reply to daejin kang
    TI__Mastermind 25705 points

    daejin,

    Thanks for your explanation. Several follow up questions: 1) when you do the measurement, the device is not operating. Is it powered with 5V? Have you ever tried measuring with Vcc on? 2) If you test a new device, do the results look like the initial value? 3) again, what's the equipment you use, a handheld multimeter?

    Yes, DS26C31 has built-in ESD cells on the output pins. However I'm not sure how they're related to your observation.

    Regards,

    Hao