Controlnet Problem (Control Logix)

If anyone has ever used controlnet....it can be a ain the a$$. Anyways...i'm having a problem....here's the deal:


Have redudant processors with an SRM module in each chassis connect with fiber.

Each chassis has a controlnet bridge with A/B channels. We also have a I/O in a remote chassis below. Client asked to have 2 redundant CNB modules in the Remote Chassis. No big deal.

So we have 1786 Controlnet T taps for the controlnet. I have assigned node addresses to each card with the lowest address being that in the i/o chassis. I have scheduled a network with Rsnetworx, and set up the keeper.

Everything is working fine; however, the redudant CNBR modules in the I/O chassis are having issues. Both A/B channels show up; however, if i remove both A/B channels of the first card, my I/O drops completely out. If i remove the A/B channels on my 2nd CNBR module, the I/O keeps.

I've traced back cables/connectors, and everything is set up fine. Network is scheduled; however, I think the keeper is being lost. The keeper starts off each time in the first cnbr module, when those a/b channels are removed from it, i don't think the keeper is being passed on.

Anyone have any ideas??

Also want to add.


I have set properties for both a/b channels and they are showing up as solid green lights on the cnbr modules. My network update time i have set for 15ms - 20 ms. The manual kind of led to not setting above 90ms if more than one cnbr module is in the same chassis.

Ok...i think i found the problem. The firmware determines if the card is a single or multi-keeper. I think our firmware revision is set for a single keeper, when we need a multi-keeper, or a keeper to pass to all CNBR modules.

Our keeper right now is being held in slot 0; therefore, when we drop it out...we loose our I/O...when we drop out Slot 1 CNBR module, we still have our I/O.

The department of redundancy department

Are there other ControlNet nodes besides these four 1756-CNB's ?

ControlLogix hot backup requires at least two ControlNet nodes besides the two for the hot backup controller pair. If you have just one I/O chassis, maybe your customer called for those two CNB's so that they'd meet that requirement.

How is your I/O configured in RSLogix 5000 ? If you are using the 1756-CNB in Slot 0 to make all your I/O connections, then the one in Slot 1 is just there to participate in the network. Such a configuration is going to behave exactly as you describe.

I think maybe that the designer intended to have the 1756-CNB in Slot 1 take over the I/O connections for that chassis if the 1756-CNB in Slot 0 failed or was disconnected.

That's not how ControlLogix hot backup works; you can have redundancy at the controller level, and at the cable level, but not at the module level. I/O module failures are just as likely as Communication module failures, so you don't improve the overall integrity level of the system by adding more than one communication module to the remote chassis.

Hi Ken. Thanks for the reply!

In rslogix 5000....the i/o is set up as the first cnbr module in slot 0 and the 2nd cnbr module in slot 1. The I/O is then below. I'm not so worried about the set up of the network. We have done two panels similiar to this one, but Controlnet never behaves the same way each time! It's a mystery sometimes

When scheduling the netork in RSnetworx, I can see both cnbr modules with defined I/O on each.


I agree with you on the way the client requested to be ridicilous. We have redundancy redundancy because each controlnet card has 2 channels that are redundant. You are correct also about their engineering of putting two cnbr modules in the same chassis. If one card goes back, they want the other card to pick up.

After the firmware flash tommorrow, I will be sure to update on the status for future controls engineers. Hopefully the info will pay off for someone else

intruder805 said:

You are correct also about their engineering of putting two cnbr modules in the same chassis. If one card goes back, they want the other card to pick up.

Click to expand...

Redundancy doesn't work that way. You would have to monitor the connection of each of the modules and route your communication through that module. If you have I/O in the "3rd" chassis, it would be configured through your I/O tree, and I'm not sure if this would be an issue for re-routing the communication.

intruder805 said:

Hi Ken. Thanks for the reply!

We have done two panels similiar to this one, but Controlnet never behaves the same way each time! It's a mystery sometimes

Click to expand...

Your observation that the behavior is different could be due to firmware revision levels. The CNB has seen many revisions in the last 3 years that fix various problems. With CLGX it is a good idea to keep notes on the firmware revisions of the modules shipped for your equipment especially on the communications modules (SRM, CNB, CNBR, ENBT, ENET, etc...) I am not certain if RS Logix 5000 keeps this information, but if it did would be a good way to check for your own sanity sake.

Just to back up what Sheldn says; you have to have very specific levels of firmware for all modules used in a CLX redundant system: have a look at this Knowledgebase document for more details. (Unless you're using V15 'cos it's not mentioned )

Oh yeah... forgot to second what NOP said. The version of Logix also matters. We are using ver 13. So am not up to speed on what version 15 does differently. But firmware and Logix versions go hand in hand and sometimes not but it all matters.