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THVD1450: THVD1450 A/B diff signal distorted at 4Mbps

wen yuanming
Prodigy 20 points
Part Number: THVD1450

Dear TI Expert,

I am design a RS485 bus with one master and one slave,  the Master side using ADM2582,  the slave side using THVD1450,  both side was terminated with 120 resister, But I encountered a very strange phenomena,  when Master send data to slave with 4M bps data rate, everything goes fine, the differential A/B waveform is good as expected, but when I send data from slave THVD1450 to master, the differential A/B signal will become distorted, and the master side can't decode the right signal 

Yellow is TXD signal of slave, the blue one is A/B differential

can you help analyze the problem and root causes of this?

Thanks

DW

  • 0
    Guru 316640 points

    Please show a schematic of the slave, and the A/B signals.

  • 0 in reply to Clemens Ladisch
    TI__Guru 53555 points

    Hi Wen,

    The A and B waveforms will help us determine if there is an issue with the driver or differential signal as a whole. From the common-mode signal, this does not look particularly problematic on its own. That said, please ensure that the connections for both A and B are secure. 

    If possible, please also include a scope shot of the system operating properly with the ADM2582 driving the bus. Showing A and B independently here would also be helpful. 

    Regards,
    Eric Schott

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Hi Clemens,

    The schematic is fairly simple, there is a 120ohm termination resister and a TVS array diode to suppress the transient noise  

    Thanks

    DW

  • 0 in reply to Eric Schott1
    Prodigy 20 points

    Hi Eric ,

    below is captured signal when send data from master to slave,  it has no problem

    YELLOW: slave RXD
    Green: master TXD
    Blue: A/B Differential

    but the problem happened when send data from slave to master, below is A/B signal by single probe, there are heavily distorted, I don't think there is a termination issue since I set a fairly simple point to point system with very short twisted wire, both master and slave side were terminated with 120ohm

    Thanks

  • 0 in reply to wen yuanming
    Guru 316640 points

    That common-mode ringing is strange, and it looks as if the THVD does not have enough drive strength.

    You did not mention decoupling capacitors. Please show the schematic of the slave's power supply.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Hi Clemens,

    The problem has been solved, we found the 485 transceiver marked "1450" is a fake one, it only can support 500kbps,  we replaced it by a true THVD1450 from TI, everything works fine now 

    Thanks for your support

    DW