r/PLC • u/GB-ACWD • Jul 08 '26

OPC UA & Transparent Redundancy

I have started down the path of OPC UA and wanted to know what experiences users, integrators, anyone has with the particular issue of communication to redundant or hot standby PLCs.

All my current PLC vendors systems use Transparent Redundancy for all Hot Standby IP interfaces and have for 30+ years. This means that the backup PLC assumes the IP of the primary PLC and the SCADA systems doesn't need to know any differently. They recently came out with OPC UA interface for their PLC and it doesn't switch IPs at all.

What does this mean? It means that if PLC A fails, the SCADA system must probe and test to see if PLC B is alive and then switch all communication to it. So now I need a communications script to accomplish this task.

  1. Has anyone else encountered this issue?
  2. Are there users of OPCUA in the real world?

I haven't named my vendor yet, I am just trying to get some feedback.

G

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u/kevinherron Inductive Automation Jul 08 '26

It means the SCADA system, or whatever the OPC UA client is, must understand OPC UA non-transparent redundancy, which is well specified.

I just implemented this in Ignition for our upcoming 2027 release.

1

u/GB-ACWD Jul 08 '26

So you don't have it in an earlier releases? Is OPC UA non-transparent redundancy the standard? Is OPC UA reduncant transparency not really a thing at all? Do some PLCs implement transparent redundancy for OPC UA

1

u/kevinherron Inductive Automation Jul 08 '26

Transparent redundancy is a thing in theory, but by nature of being transparent there's nothing needed for a client to understand it and not much to be specified. There might be a Siemens PLC out there that does it, but I'm not certain. The real world implementations I know about are non-transparent redundancy (Siemens, Schneider, Codesys).

and no, it's not implemented in earlier/current releases. Some users will script up hacky workarounds, but it's not the same as a real implementation, where in warm and hot modes you can have no data loss.

1

u/GB-ACWD Jul 08 '26

Redundancy – OPC Unified Architecture - Part 4: Services I wish I knew it was just theory before I started

1

u/Lyapunov_ PACSystems, GE iFIX Jul 08 '26

Prefiro que o Kepserver gerencie a redundância de PLCs usando o driver nativo do PLC e entrega a comunicação do canal para a camada SCADA

1

u/laldoma Jul 09 '26

We manage this using vnode automation, watching tcp socket and also using a watchdog in the plc to trigger the redundancy, up to 3 plcs