The setting: network with about 10 PCs and 8 AB RSLogix 5000 PLCs.

The issue: Everything is communicating OK, but I'm seeing what appears to be a lot of broadcast traffic within the network. It is noticeable because the activity light on every connected port on every network switch flashes in unison. In normal circumstances, you expect to see each device's activity light flashing randomly to indicate its own activity, and a network switch is supposed to route traffic between sender/recipient to reduce overall network load.

It appears to originate with the PLC side of the network, because disconnecting the switch with the PLCs from the rest of the network stops the synchronized blinking except on the PLC switch.

Four questions:

  1. Is broadcast network traffic common with PLCs?
  2. If so, what is its purpose?
  3. Will this negatively affect network performance?
  4. Suggestions to improve this issue?

I plan to run Wireshark to determine exactly what's going on, but wanted to ask for advice here also, as I'm sure some of you have seen this before.


  • \$\begingroup\$ I may have found part of the answer here: forums.mrplc.com/index.php?/topic/… in the statement by "paulengr" about halfway down the page. \$\endgroup\$ – Ryan Griggs Feb 5 '16 at 2:12
  • 1
    \$\begingroup\$ You don't have any physical circular loops formed between the different networks, correct? One misplaced cable (forming a circular data path) can cause digital feedback which could appear as continuous multi-cast. Otherwise, replace the switch with a router that has QoS / IGMP / VLAN capability and isolate the PLC devices by placing them on a different subnet. \$\endgroup\$ – rdtsc Feb 5 '16 at 12:51
  • \$\begingroup\$ No I don't think so. I just replaced all the networking components with upgraded switches, patch panels, etc. Everything looks like standard star topology. It appears in the article above that PLCs sometimes do send broadcast packets if set up as sender/receivers. This must be what's up. But I have to install RSLogix and look at the individual PLC configs to see for sure. \$\endgroup\$ – Ryan Griggs Feb 6 '16 at 17:23

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