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SN65HVD3085E: Bus voltage issue

Part Number: SN65HVD3085E
Other Parts Discussed in Thread: THVD1550, THVD1429, THVD1450, THVD2450

Hello,

We use the chip on one of our aviation controllers.

We have had issue with two of our controllers where the bus was transmitting data but the voltage level was <1V causing the board to fail.

When we replaced the transceiver IC, the controller started working again and no issues were encountered.

We are not able to come to the root cause of this issue. Can you help with that?

Thank you!

  • Hello Manushi,

    Is it possible the chip became damaged during operation? (Data ports are frequently the victim of coupled transients since they interface externally to the PCB.)

    Did you happen to capture any signal waveform when the issue was present that we could review?

    Regards,

    Max

  • Manushi,

    Do you have any updates on this issue?  Is it resolved?  If not, have you had a chance to review my comments above?  Please let us know how we can help.

    Regards,
    Max

  • Hi Max, 

    We have lost the part so we were not able to capture the waveforms. But we have another part which has a similar failure. I am attaching the screenshots for those. 

    This part though has a shorted 5 V. I put the part on another board to make sure it isn't a board issue, but still no 5 V. 

    Can you help us in understanding that what could have possibly gone wrong? 

    We see 200 mV on pin 6 of the IC and about 5V on pin 7.

  • If you could share a schematic of your usage of the chip it may help.  Based on the waveforms, though, it looks to me like there may be increased leakage to ground on pin 6, and this is resulting in the driver having some difficulty in driving a high output level (due to the increased loading).  This is a common failure mechanism of electrical overstress, which itself is a common failure type of RS-485 buses due to their exposure to transients and faults that could be induced on a connector port or cable.

    Most commonly this sort of damage results from transient overstresses like electrostatic discharge.  If that is the case, you may want to use a transceiver with a higher level of ESD immunity integrated.  A p2p example would be THVD1550 or THVD1450 (if 3.3-V operation is desirable).  For even higher levels of transient immunity, you could use THVD1429.  Or, another option is to use an external TVS diode like SM712 for protection.

    If there is a risk of the communication lines being shorted to a higher-voltage power supply, protecting the transceiver may be a little more complicated.  In that case, the easiest solution is usually to use a device that is rated to support high voltage faults like THVD2450.  Or, high-voltage protection can be implemented discretely using components like PTC fuses.  An example implementation is given in this reference design: https://www.ti.com/tool/TIDA-060027.

    Regards,
    Max

  • I should mention as well that if the root cause is damage to pin 6, you could verify it by checking the input leakage current to the pin and comparing it against the bus input current specification in the datasheet.  The easiest way to do this is by applying a known voltage to the pin and using an ammeter to measure the current.  Or, if you don't have an ammeter you could apply the voltage through a resistance (like 1 kOhm) and compute the current based on the voltage drop across the resistance.

    Max

  • Thanks Max for the detailed reply. I will measure the leakage current. Please find attached the schematic. 

    Thank you,

    Manushi

  • Manushi,

    Thanks for the schematic.  Overall this looks OK to me.  I see there are no protection circuits at the J3 port, though, and so the most likely root cause of this issue still seems to me to be damage to the interface pins due to stresses coupled from the connector/cable.

    Max