ControlNet for Redundancy system in ControlLogix

Hi Everybody, I have a question regarding ControlNet in ControlLogix redundant configuration. I have two ControlLogix PLCs working in a redundant configuration and ane remote I/O which is FlexLogix I/O. ControlNet is used to connect PLCs to Remote I/O, so I have two CNBs in two redundant chassis and one ACN15 in remote FlexLogix rack. I already know that I need to add one ControlNet node to satisfy requirement of minimum four nodes. My question is: what kind od ControlNet device do I need to add? Would another ACN15 be OK? ACN15 is not capable of beeing a network keeper, so in such case one of redundant CNBs would have to do this. Based on my rather extensive conversations with AB Tech support, is is recommended but NOT REQUIRED that network keeper be outside on redundant chassis, in other words CNB module in redundant chassis can be a network keeper(knowledge base answer no. 60555). Other engineers however (as well as other knowledge base answers: 41441, 45930) strongly suggested that network keeper node should be outside of the redundant chassis. I spent so much time on AB tech support line, still do not have a definite answer and am little bit frustrated, so I decided to try this forum hoping that somebody with hands-on experience can help me, any hope or suggetion will be greatly appreciated, Thanks in advance, wesley_s

As I understand it, the function of the ControlNet devices that are at a lower node number than the 1756-CNB/CN2 modules in the Redundant Chassis is the Moderator function, not the Keeper function.

You can use any ControlNet device for this role. I've even done it with RSLinx and a -PCC card (during a demo). I have seen users buy just a 1794-ACN for this purpose.

Ken, Thank you for the reply. So the lowest node number has to do with network moderator rather then network keeper. So the lowest node number becomes the moderator (this node does not have to be a keeper-capable) and the lowest node that is a keeper-capable (that includes one of redundant CNBs) takes a role of a network keeper. So in my case, if I add one 1994-ACN15, which is not keeper-capable, but can be a moderator (i.e. provide the heartbeat on the network) and assign node numbers, for example 1794-ACN15 node #1, second 1794-ACN15 node #2, redundant CNBs nodes #3 and #4 it should work - is this assumption OK?

knowledge base answers: 41441, 45930) strongly suggested that network keeper node should be outside of the redundant chassis

Click to expand...

These TNs never mentioned this - I searched for word "keeper" in both and did not find it

The bottom line: two additional nodes don't have to be keeper capable. Check user manual - it clearly states this:
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um523_-en-p.pdf page 28
An additional node can be:
• a second CNB module in the same remote chassis or in a different remote chassis.
• any other ControlNet device (not required to be keeper capable).

Thanks!

What would happen if the lowest node number device were to fail? Would the ControlNet network fail?