We’re working on an architecture for a SCADA system that ends in the PLC being the only single point of failure. Naturally we will be using an Ignition Edge local client fallback setup with a central Ignition Gateway.
- Store and Forward on PLC tags (and so obviously history on those tags)
- Failure of the central gateway or connection to the gateway will maintain machine control and result in no data loss after the connection is re-established.
- Failure of the Ignition Edge gateway will maintain machine control and result in no data loss after the connection is re-established.
The tricky part is requirement 3. We don’t want a hardware failure of a Edge node to be the reason a system can’t log data or interact with the rest of the SCADA system.
The solution so far:
- Add the PLC as a device on the central gateway and the edge gateway
- In the central gateway, create a local tag provider for the direct device tags
- In the central gateway, create a remote tag provider for the device tags coming from the edge gateway
- Both tag structures in 2&3 match.
- Create a 3 tag structure of purely expression tags that references the preferred tag provider based on edge gateway connection status.
This solves the machine operation needs of requirements 2 and 3. The trick is then store and forward.
- You could store history on both providers (from items 2&3 above); but then there’s no way (that I know of) to synthesize the history after an edge node comes back online.
- You could do history on the expression tags, but then you would lose all store and forward, at least as I understand it. If I’m wrong please tell me.
Anyone helpful ideas on how to solve this one would be greatly appreciated. Thanks!