Redundant ControlLogix PLC control net network

chapani

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Jul 2014
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Dear all,

I have just arrived to a new site and found very strange Control net network.
Just to explain the situation. there is redundant CLX PLC system, where each (Primary and redundant) PLC have three CN2R/B control net cards, each one has redundant A and B port. The network A is connected as following all three CN2R cards are connected to the same controlnet network plus other three A from Redundant PLC, by interconnecting Taps and then is one RPA with 4 RPFM modules and later on is 8 branches of controlnet networks connected with fiber.... Same thing is for network B on other side, so everything is symmetrical.

Inside RSLogix software under 1st CN2R there are 12 ACNR15 flex I/O, under 2nd other 10 RIO flex I/O, under 3rd 6 flex i/O rios, 3 CNBR and 1 CN2R RIO.

Physically the networks are not divided as it is in RSLogix, s all three CN2R cards can reach all nodes as they are connected to the same network.

My problem is that if I disconnect one node from the network A, or if I disconnect controlnet cable from ch A of any CN2R cards, I will lose complete A network, same thing is with network B.

if I disconnect another node (not same one) from B network i will lose redundant PLC it fails out from Sync... and all network start to blink...

As it is first time that I see this kind of configuration with connected all CN2R cards to the same network, I was wondering if this is the problem, should I split it and rearrange all rslogix project to have three different networks with exact nodes that can be reached from each card under each card.

thank you
 
I uploaded the rack configuration and the sketch of Control Net network.

All three CN2R cards are connected on same Network A or B in U1-1 cabinet.

thank you for your assistance

rack.jpg
 

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The Channel A/B behavior and the Redundant controller Sync behavior you are describing are normal.

ControlNet media redundancy works by selecting Channel A or Channel B to be active. If the non-active channel is ready to take over, then both channel LEDs will be green.

But if one device is disconnected from one channel on the network, all the devices know about it and they'll declare that channel to be faulted because that channel can no longer provide communications to all of the devices on the network.

Dual-channel devices like the 1756-CN2R cannot make some of their connections on Channel A and some of them on Channel B. So if it can only communicate with one or more of its scheduled devices by using the inactive channel, then it will break the connection, and that will make the Redundancy system not ready to switch over.
 
Thank you for posting that thorough and detailed network diagram and the I/O list.

My guess is that the three CN2R pairs all connect to the same ControlNet network because some of the connections are only reachable by a certain fiber and copper path.

But the diagram isn't complete; there are six un-addressed devices, and the 1756-CN2R pairs that show in the I/O tree (39+40, 41+42, and 43+44) are not shown in the network diagram, and there are multiple devices in the I/O tree that are not shown on the network diagram.

This looks like a network that could be segmented in a way that would split it into three separate ControlNet networks by moving connections in between the three root bridge 1756-CN2R modules.

For example, Node 14 and Node 15 are 1794-ACNR15 adapters located at the U2-2 location and connected to the same fiber and copper ControlNet media.

But Node 14 is logically connected to the 1756-CN2R in Slot 2, but Node 15 is connected to the 1756-CN2R in Slot 1.

There might be other considerations for bandwidth or inter-controller produced/consumed tags.

I try to simplify and segment things for maximum fault tolerance and performance while retaining the speed and functionality the application needs. Your system might benefit from that approach. At a minimum you'll need some additional 1786-RPA modules and some room in the U1-1 enclosure, and some careful adjustment of the ControlNet network signal tuning parameters.
 
Ken,

thanks for answer.

I haven't showed on Digram 6 CN2R as they are all connected together on network A and B as I explained before. it is all one big network.

I would like to know if I split network in 3 redundant networks, and reorganize little bit Control logix project, to put under correct CN2R only nodes that can be reached directly from it, via fiber and control net, will I avoid losing complet network A or B if one branch of network is disconnected, and will it be easier for troubleshooting. Because at the moment is nightmare, i don't know where to start to look for the problem, as when I disconnect 1 node from network a or B I lose complete network A or B.

any suggestion is welcomed
 

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