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SN75176B: RS485 network failed

veeraswamy kakita8
Intellectual 320 points
Part Number: SN75176B
Other Parts Discussed in Thread: THVD1510, SN65HVD1786, SN65LBC184

We are using SN75176B for MODBUS rs485 communication with Isolated DC supply. Earlier we used Zener diodes for protection at A and B, in some cases these zener diodes getting shorted or opened due to this entire network getting stopped communication with Master.

Now we removed all zener diodes and replaced SM712, here we observed that in one of device in a network B and Ground pins got short and the same device communicating with Master and working fine and remaining all are failed to communicate with master and remains in the same position (Hanged). After removing the failed device from loop then the remaining all are working fine.

Can you please suggest any solution for the failure.

  • 0 in reply to veeraswamy kakita8
    TI__Expert 3810 points
    Hi,

    thank you for reaching out! Correct me if I'm wrong, but apparently after introducing the SM712 the only fault that you can see is the B line being shorted to GND. This helps to understand where the fault could be generated but it's still not clear for me. As a matter of fact, on the B pin there are several components connected to; I would kindly ask you to verify if it could be one of these diodes shorting the pin or if it's our device.

    In order to do so, I would suggest to test first the system without the transceiver, then test the transceiver without those diodes. This kind of test could help to understand if the problem relies on the device itself or on what's on the PIN.

    In addition to that, could you please provide a schematic with a better resolution? The one attached here has a very pixelated view.

    Thank you in advance and best regards,
    Adrian
  • 0 in reply to veeraswamy kakita8
    Intellectual 320 points
    No other component failed in entire circuit, only SM712 pins got shorted afterwards there is no communication between master and slaves and this leads to entire network failure.

    After removing failed device from network and entire network working fine
  • 0 in reply to veeraswamy kakita8
    Intellectual 320 points
    As you said we disconnected SM712 and all other components from circuit and we found SN75176 7th and 8th shorted with 30ohms.

    Can you please explain why it is shorted and what are the precautions we need to follow.
  • 0 in reply to veeraswamy kakita8
    TI__Expert 3810 points

    Hi,

    thank you for re uploading the schematics. It's good to know which part of the system is failing thanks to your test. I do have a suspect that the cause of this failure could be related to high transients on the B line. Could be that the operational values on the B line are exceeding the maximum tolerated values, thus making the device failing.

    I do have a question about which pin gets shorted because in the last post you mentioned pin 7 and 8 shorted, that is B line to Vcc; while in the first post you suggested the B line was shorted to ground. Could you please clarify that?

    It would be helpful for me to also understand your system condition: what values do you use and do you have any over-voltage? Would it be possible for you to share the waveform from the 485 line?

    So far, I would suggest you to take into consideration newer devices such as the THVD15xx family. As a matter of fact, having a device which can sustain high ESD phenomena and a wider range of voltages on the bus line could help to avoid the pin being shorted. Depending on your specific needs, I would suggest the THVD1510. If you could share more info about your application, I could suggest a more specific device.

    Let me know if this helps.

    Best regards,

    Adrian

  • 0 in reply to Adrian Ymeraj
    Intellectual 320 points

    These devices are used to monitor environmental parameter, control parameters and communicate with BMS using RS485.

    This is for pharma industries and HVAC automation projects.

    While using SN75176 we connected SM712 which is also transient protection device, in our present scenario SM712 is healthy but SN75176 B(7th) pin shorted to Ground only (Not with Vcc). 

    Can you please suggest how much comfortable we can you this THVD1510 for this type of applications.

  • 0 in reply to veeraswamy kakita8
    TI__Expert 3810 points
    Hi,

    so the device is used for multiple applications. Do you still have the same kind of requirements? Because depending on what the needed specs are, I could suggest a more precise choice. Overall the THVD15xx family, namely the 1510, could be a good fit for your type of applications since the signal rate isn't very high and they offer good ESD protection and an extended common mode range on the pins.

    Another recommendation would be the SN65HVD17xx family, namely the SN65HVD1786, again offering protection from ESD events and a wide common mode range (-20/+25V).
    We do also have a special part with transient voltage suppression called SN65LBC184, I would suggest to give it a look.

    Let me know if this helps.
    Kind regards,
    Adrian