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  1. #1
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    Nov 2006
    Harrisburg, PA, USA

    Modbus TCP out of order packets, duplicate ACKs and connection problem


    I am using Modscan32 as my Modbus TCP/IP Master host.
    I am communicating with modbus slave over ethernet.

    I configured modscan to poll the data every 5 seconds.
    After some time, I saw TCP IP duplicate ACk's getting sent by slave. Modscan32 respond to these ACK's with ACK's. This process seems to be happening in a loop.

    Sometime I also see "Modbus Message Overrun" message on Modscan screen.
    Sometimes Modscan host sends multiple Modbus TCP packets within 1 TCP/IP packet. On the slave side I have linux TCP/IP stack and modscan32 host is running on Windows machine.

    PFA Wireshark/ethereal .pcap file for the transaction.
    Modbus host IP address:
    Modbus Slave IP Address:

    Please let me know how should I fix this.

    Attached Files Attached Files
    Last edited by sumitv; 04-27-2010 at 12:59 PM. Reason: Provide additional information

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