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SN65HVD1782: RS485 communication cannot work after sometimes

Poh Kok Keng
Genius 5512 points

Part Number: SN65HVD1782

Dear Sir/Madam,

I am using SN65HVD1782D in my design, the project already in mass production. But recently we faced some issue on RS485 communication is not stable. After sometimes, RS485 cannot work.

This happened to the TOP IC marking: VP 1782 07K A39X G4.

I am using baud rate of 9.6Khz at Vin= 3.3V/ 5V, Vcc = 3.3V. 

Pin6 (A), no output

D9 MPN is SD103AW-7-F. is this protection enough? 

Here are some measurement of the good and bad waveform. 

Bad waveform: CH2 485-A; CH3 485-B

Good waveform: CH2 485-A; CH3 485-B

Do you have any advice why RS485 initially can work and after sometime, it will fail to work? any improvement need on the circuit design? 

We need to find out the root cause. 

Best Rgds,

Felix 

  • 0
    TI__Guru 53852 points

    Hi Felix,

    Thanks for reaching out!

    I think the problem is the diode - it is not going to protect this device. 

    Its working clamping voltage is 40V the SN65HVD1782 is only rated to 30V:

    There is one more thing to note - that under normal operating conditions this device is still only recommended to have the bus common mode voltage (A to gnd and B to gnd) between -12V and 12V and we don't recommended planning a design where that could be violated under normal conditions (it still is fault protected) - we also have these devices with extended common mode ranges if necessary - datasheet here:  https://www.ti.com/lit/ds/slls872i/slls872i.pdf?ts=1649175085613

    Please let me know if you have any other questions!

    Best,

    Parker Dodson