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I've been scratching my head with this one and am hoping someone has dealt with it before. My physical setup is a PC, CAT5 cables, a couple of switches, an Eaton HMI, and an Eaton PLC. I'm talking Modbus TCP to the PLC using the library on NI's website. I'm using Wireshark to troubleshoot other problems and found the PLC retransmitting the response to the PC very frequently (not every time). The PC is reads and writes 32 words to the PLC every 100 msec, but only performs a write when the data changes. The PLC transmits a response within 6 msec of a read message (function 3 in Modbus). The PC