A-B Hot Backup - what's your opinion?

I'm getting what appears to be more requests for hot backup using Allen Bradley PLC and SLC platforms. I have my own opinion regarding the need for this but I'm curious what the collective school of thought is from the experts here at this site.

AB has different solutions to redundant processors, depending on the platform and communications protocol chosen. In the SLC line, the 1747-BSN module lets you switch RIO or DH+ to the active processor along with the RS232 link for an LOI. In the PLC line, the 1785-BCM gives you the same functionality. All this works well as long as the LOI is using DH+ or serial links since the backup module handles the redirection and no addressing changes are involved.

However, when using controlnet backup with the PLC line, the controlnet address of the two processors is different, so the LOI can't easily switch to the active processor when transfer occurs. The exact same problem exists with Ethernet on either processor line. How have you gotten around this? And is anybody really doing much hot backup these days?

Sorry for such a long winded question. The local AB folks haven't been able to give me very firm guidance on this either.

I guess I would start by asking your customers as to why they think they need hot backup in the first place...I have not done any research on it, but the topic does interest me and hope to see some knowledgeable posts on the topic...We are gaining more and more ControlLogix processors in our plant.


Good luck,

Greg

I don't know too much about SLC or PLC5 hot backup systems, but did some ControlLogix applications.

All versions of CLX redundancy (8, 11, 13) will do CNB node swap during switchover.

With ver 13 of CLX redundancy incorporated automatic IP swap:
during switchover ENBT or EWEB will swap IP address with its partner in redundand chassis.
HMI connected over Ethernet will experience very short timeout, but will continue to work normally.

With PLC-5C15 ControlNet hot backup, the Primary and Secondary controllers do retain their ControlNet node addresses no matter what mode they are in.

RSLinx handles this with an Alias Topic, which can switch between target nodes based on failure or on a bit change (like the IsPrimary/IsSecondary bits in the controllers). The HMI software can use RSLinx OPC Server and just rely upon the Alias Topic to point to the Primary controller. This works over Ethernet or over ControlNet.

IMHO, the PLC-5C15 ControlNet Hot Backup is the only way I would implement a redundant PLC-5 today. Diagnosing and commissioning those old 1785-BCM were a big headache compared to the way the ControlNet system works.

I agree with Greg - find out what the customer thinks they need and work through if they really need that functionality.

I have worked with pretty much all of them, and definitely prefer the ControlLogix redundancy. My favorite part is that there is no dedicated scenario for primary/secondary. Either one can be the primary and the other secondary.

For example, using PLC5 with the BCM, you have a fixed primary and secondary - if you swap and the secondary becomes the controller, the previous primary is dead and is not the secondary. Funny story - the customer thought that they needed redundant processors because they were monitoring compressed air (their primary utility). All they were doing was monitoring, no control. Not a necessary application for monitoring.

My experience in the past with these things is that, it is a nice to have thing that ends up in the bid specification.

During the contract letting phase, its almost always the first thing to be deleted. Mostly, because the end user can't remember why it ended up in the spec, or can't properly justify the cost.

And, considering all the single-point failure modes, the processor and i/o systems are likely the last to break. Field devices are more likely the issue, normally.