Problem with DupAcks of Modbus TCP

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  • #6418
    Avatarvishnuvardhan
    Participant

    Hi

    I am using Rapid Scada as Modbus TCP Client. I am polling 20 registers at a time, a total of 100 registers in a sequence once in a second. Whenever the server sends DupAcks to the client the sequence of polling is getting changed and I am receiving wrong values(which I verified by polling the same registers with some other TCP client software). So, here my doubt is whether the Rapid Scada is capable of handling DupAcks. If it is capable why i’m getting this issue.I’m getting incorrect PDU length in communication log.
    https://drive.google.com/open?id=1kgGytct6VpXHIVkTtcOQ5Fuhz2rf30N6
    Awaiting your reply..!!

    Regards,
    Vishnu.

    • This topic was modified 8 months, 1 week ago by MikhailMikhail.
    #6420
    MikhailMikhail
    Moderator

    Hi Vishnu,
    Please post the same log as a text (not image), so I can check it using a tool.
    You can find the file in C:\SCADA\ScadaComm\Log

    #6421
    Avatarvishnuvardhan
    Participant

    Hi,
    I am posting the log below.
    https://drive.google.com/open?id=1rRHtrDnQDPsl3vIkRI4ahcXMauD2LahT

    Regards
    Vishnu.

    #6422
    Avatarmanjey73
    Participant

    It feels like your device is sending out packets again

    #6423
    Avatarmanjey73
    Participant

    Here you have a request for 5 registers and returned 5

    Request element group
    Send (12): 00 00 00 00 00 06 01 03 01 38 00 04
    Receive (7/7): 00 00 00 00 00 0B 01
    Receive (10/10): 03 08 FF ED 00 13 00 00 00 00
    OK!

    And here you have a request for 10 registers but returned 5 from the previous request. And so all the time

    Request element group “offet and gain”
    Send (12): 00 00 00 00 00 06 01 03 01 63 00 09
    Receive (7/7): 00 00 00 00 00 0B 01
    Receive (10/10): 03 08 FF ED 00 13 00 00 00 00
    Incorrect PDU length!

    #6424
    Avatarvishnuvardhan
    Participant

    Yes, I noticed in wireshark that some TCP Retransmissions have happened and few DupAck’s also. After that the polling pattern has changed, It used to poll at addresses 110,124,138,163,13C before TCP Retransmissions but after that it is polling at same address for twice or thrice continuously and at other addresses it is polling once. Why this would have happened? If I close the connection and re-establish then it’s working fine until some TCP Retransmissions occur.

    #6425
    Avatarvishnuvardhan
    Participant

    If I poll the same addresses using some other Modbus Master Simulator it is working fine..!!

    #6426
    MikhailMikhail
    Moderator

    In the properties of communication channel untick the checkbox “Stay connected”.
    Increase the timeout and delay parameters of the device.

    #6447
    Avatarvishnuvardhan
    Participant

    Yeah, I will do that but I want to know what is the root cause for this problem. Sorry for the late reply..!!

    #6449
    MikhailMikhail
    Moderator

    Root cause will be clear when the problem is solved.

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