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SN75176ADR IC failure

naveen bn
Prodigy 30 points

SN75176ADR

i have observed the issue in the above mention part (RS485 communication IC) rx pin should be default high in the normal working but it is default in low but if i replace with new IC then it's working as expected (default high state)

and D pin is default in high state in both working and non working IC also

what is the issue in the IC for this error

  • 0
    TI__Intellectual 2885 points
    Hello Naveen,

    I'm looking at the data sheet (pg. 10,11) and it seems like the RX goes high when there is a positive voltage applied, and low when a negative voltage is applied. I'm assuming you have the same test conditions for both ICs.

    How many chips have you tested? What is the ratio of "good" to "bad" chips you have tested? It may be that the original chip was damaged. Also, have you tried switching to another RS-485 chip?

    I'm also looking at page 13 section 9.3.2 and it seems like the output goes high when Vid is greater or equal to 200 mV.

    What is the input voltage you are using? The root of your problem might reside in your Vid (differential input voltage). Reference Table 3, on page 13. when the Vid is greater than 200 mV, R output is High, and when Vid is smaller than -200 mV R output is Low. The problem could be in the in between area

    -200 mV < Vid < 200 mV. When your Vid is in between those values the output is unknown.

    You could also implement a RS-485 failsafe biasing circuit to force the Vid to be at an appropriate level.

    Regards,
    Matthew Mendez

    Applications Engineer
  • 0 in reply to Matt Mendez
    Prodigy 30 points

    Specification:

     input supply(Vdd): 5V,

    we are using these IC in our products and out of 5000 units we got 15 number like this issue.

    if we replace the IC with new IC in the same UNIT then it working fine as per the expected.

    refer the schematic below.

    enable (2,3) driving from MCU.

    let me know if more information required.

  • 0 in reply to naveen bn
    TI__Intellectual 2885 points

    Hello Naveen,

    I believe I have found a potential solution. Using your pull-up/pull-down resistors I was able to calculate what the Vab voltage would be, it was around 0.13 V. This is inside the null region between -200 mV and 200 mV. This could be why some parts appeared to not work. I calculated the maximum resistor you need to enable to differential voltage to be 200 mV, and it is 1.4 k Ohms. I would recommend using 1k resistors to have some tolerance. 

    Best Regards,

    Matt

  • 0 in reply to Matt Mendez
    Prodigy 30 points

    Hi matt,

    Thanks for your effort to finding the issue in the pull up and pull down resistor.

    after replacing the resistor device start working as expected.

    Best Regards

    Naveen