ControlLogix and ControlNet unable to restart afer a fault

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We have 2 x 1756-L71 Processors joined together with a pair of 1756-RM2/A redundancy modules.

The rack also has 1756-EN2T and 1756-CN2R modules. 


I was called to the site after a power outage and found that the link between the 2 CPUs had failed with one displaying no program and the link modules with error lights.

The ControlNet cards also had red lights on both channels and all of the FlexIO devices were red Xed in RSLinx.

I went down the path of trying to get the CPUs up, and running but this was a red herring as I was informed that if the CPU's get out of Sync and have any faults on the cards the secondary one will lose its project.

The ControlNet cards were giving a no Active Keeper fault?

So we went down the path of the ControlNet cards, I initially thought that it was that the cards had lost their configuration files but we isolated the coax network to just the 2 cards, and the CP's and ControlNet cards came back up online.

We joined the network back onto the ControlNet cards and found a faulty 1794-ACNR15/CFlexIO module.

Once we replaced this the other devices (48 devices on the network) were not being discovered on the network.

We went around and power cycled the IO units and then they started to Auto discover and the site came back up - Note CPUs were in Program mode at the time and I flicked them to Run and the site came back online.

Does anyone have any information on why they needed to be power cycled and is there likely to be an underlying issue other than the single failed module pulling the network down?

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It has been eons since I worked ControlNet but if memory is not too rusty the following is true or close to true.

1.  ControlNet is based on a “token-passing” bus control network and if the token was held by the Flex Module that failed when it failed then the whole net does go down.

2. The 1756-CN2R was in Program Mode and not attempting to scan the net when the PLC CPU was in program.  You might have avoided the PDPU of the I/O by placing the PLC in RUN first.

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