BigJoeFourie

Problem has been solved!! Very interesting findings.... The TSX ETY - 410 ethernet card on the premium PLC configuration has the option to use either Ethernet II or IEEE 802.3 as the protocol. It seems that Ethernet II is the newer of the two. The Ety Ethernet card was configured to use IEEE 802.3. This results in the format of the ARP responce being different and it seems that the 1769-L24 compactlogix ethernet card does not support IEEE 802.3. I changed the protocol on th ETY card to use Ethernet II. Now the CompactLogix recognises the ARP responce and the connection works! It's been a while since I had to do so mutch Wireshark low level digging! Thanks and I hope this helps some other poor sod in the future.

I am using the sample logic provided on the Rockwell website to do modbus tcp with ladder logic on a new CompactLogix L24. The code works well and connects to PC based modbus client simulators. But when trying to connect to a modicon tsx premium PLC via ety Ethernet card then it won't connect. From a PC based modbus client I get an immediate connection to the modicon plc. I used wireshark and noticed the following. The PC does a ARP request to the modicon PLC to get the Mac address and the modicon responds before the connection is established. But when the Allen Bradley PLC sends out an ARP request to the modicon then the modicon responds but the Ab plc doesn't see the responce. This is why the connection is never established! The Allen Bradley ethernet card can be accessed by web interface and this shows that the Mac address for the Modicon ip is never updated in the arp table. Can anyone explain why the Allen Bradley can't see the Midicon ARP responce? The 2 plc's are located right next to each other on the same switch and subnet. I see in wireshark that the PC ARP message length is different to the one sent by the Midicon.... I would have expected the messages to be the same length? I find this quite strange to say the least!