Ethernet arp request issue

[BigJoeFourie 0]

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! Edited 23 Jul 2013 by BigJoeFourie

[Ken Roach 194]

Remember that the ARP request is a broadcast, while the reply is unicast, so it will be filtered by the Ethernet switch and only delivered to the destination IP, not necessarily to your computer. You will see both while running Wireshark on your PC, because Wireshark is monitoring your Ethernet port. You will not see the Modicon's reply to the ControlLogix unless you are running a mirrored port or a hub that repeats all traffic. If you can post *.PCAP files ? Somebody who is more expert than I am might be able to tell if there is any important difference the two ARP requests. Have you tried using the PC based Modbus program as the target of the CompactLogix message ?

[BigJoeFourie 0]

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. Edited 23 Jul 2013 by BigJoeFourie