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ISO3082: Application issues.

Reed
Expert 1910 points
Part Number: ISO3082

Tool/software:

Hi Team,

Use ISO3082.

Use ISO3082DWRG4 to match Siemens PLC - SIMATIC S7-200 SMART live unit network communication.
There is a problem of 485 communication crashing directly after the motor controlled by the frequency converter starts, and at this time, it is not possible to read and write instructions through the PLC.
Actual observation of the waveform reveals that RXD emits a significant amount of abnormal low-level logic (the default H level is frequently pulled to 0V abnormally, with a pulse width<50ns and a period of approximately 25us).

Could you please help analyze why this phenomenon occurs? How should we solve it?

  • 0
    TI__Mastermind 29375 points

    Hello Reed, 

    Thank you for reaching out. based on the schematic there is no clearer cause for this type of issue. I would recommend disabling the receiver and see if the problem is still happening. 

    Could you please identify which channel corresponds to each signal in the scope captures?

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 20 points

    Hello Andrew,

    As we see, channel 1 is A to B, Channel 2 is RXD.

    Can the AB differential level turn on the receiver of ISO3082 to work for this pulse width（＜50ns），Then it leads to the emission of RXD fake data?

    Another question，How long does the hysteresis  Comparator of receiver have a delay?Is its (high-level) threshold operating voltage 200mV？

    Thanks,

    Duan

  • 0 in reply to Duan Liangliang
    TI__Mastermind 29375 points

    Hello Duan, 

    The AB level cannot turn on the receiver. Please keep in mind that if side 1 and side 2 have isolated power and grounds, there should be no interaction between the sides from ground noise. The Receiver hysteresis characteristics are shown in the datasheet below. 

    Have you tried testing when the RE pin is disabled? There seems to be a lot of noise in the system. If the receiver is enabled in the presence of noise, it is possible that this is causing the receiver to toggle. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 20 points

    Hello Andrew,

    Thank you for your reply, we have tested the following situations：
    1.when the RE is disabled With the Waveform1 below.It looks like a simple glitch, not enough interference to pull down the RXD level to 0V.
    2.When the connection from ISO3082 to both the PIC and the further RXD signal is cut off, as shown in the second waveform below, it can be seen that in an environment with interference noise,which is introduced from the isolated communication ground (GND2, pins 9/10/15）,the RXD pin of ISO3082 has abnormal working conditions and emitted abnormal data, which interferes with our control and makes the control attempt to recover after the next communication cycle, or even completely fail.

    May I ask if you have encountered such a situation before? And how was it resolved?

    CH1:5VISO to GNDISO

    CH2:A to GNDISO   CH4:B to GNDISO

    CH3:RXD (3082-PIN3）

    the pink channel:A-B

    Thanks,

    Duan

  • 0 in reply to Duan Liangliang
    TI__Mastermind 29375 points

    Hello Duan, 

    I see the noise on the A and B pins. This noise is likely the cause of the failed transmissions since they will be interpreted as signal. The best way to fix your issue would be to find the source of the noise and stop the noise from propagating to the bus. If the source of the noise cannot be found, then it will be necessary to filter it out.

    Best,
    Andrew