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SN65HVD3088E: Abnormal RS485 transceiver Output of SN65HVD3088E

jkw
Prodigy 100 points
Part Number: SN65HVD3088E

Hi,

In my design, SN65HVD3088E is used for RS485 communication between MCU and Encoder. Date rate is 2.5Mbps fixed.

I captured the A/B signals ans Receive data output of SN65HVD3088E  in scope. please check below,

The front part of Receive data translated by SN65HVD3088E is incorrect, but the latter part of Receive data is correct. 

please help investigate this issue.

Thanks.

  • 0
    Guru 244020 points

    The rounded edge at the beginning looks as if the R output is high impedance, and the line is weakly pulled low with a resistor.

    Can you check whether RE is actually low at that time?

  • 0 in reply to Clemens Ladisch
    Prodigy 100 points

    Hi Clemens,

    thanks for your hint.

    I checked the ~RE at that time,  it was high level. It means  SN65HVD3088E was sending data at that time while the  ~RE was high-Z by the chip. 

    So the abnormal waveform(low level part) must be pulled low by external circuit.

    I checked my design and found the pull-down resistor in  ~RE  line.(there was a level translator by two resistors) 

    Finally I added pull-up resistor for this signal line and the data was good as expected.

    Thanks again.