S7 driver, Redundant Servers and _online.._bad

[n_lev]

Hi,

We're experiencing an issues and I'm not sure whether this is expected behaviour or a bug.

Here's the simplified architecture first:

I've got 2 redundant servers (A and B).
I've got several clients connected to both servers.

I've got a redundant S7-400H PLC, both servers connected to it using S7 driver.
I've got few DP's that are polled from the PLC (with "low level comparison" enabled).

I've configured an error weight on "_PLC.ConnState" datapoint, so that if active server looses connection to the PLC, the system would switch over to another server.

While all comms are OK, I've got no problems. I can manually trigger a switch from server A to B and back and everything works fine.

Next thing, I break the network connection between Server A and the PLC. After a timeout, Server A gets higher error weight and the system automatically switches over to Server B. The problem is, thought, Server A S7 driver actually sets "_online.._bad" bit on all my DP's and they stay like that until I deliberately change value in the PLC (or un-tick "low level comparison"). I can clearly see that Server B communicates OK, I can actually issue commands and they go to the PLC, but all my DP's are still marked as "bad". This actually makes my GUI show values as invalid.

So here is my question. How is this redundancy supposed to work? Isn't Server B supposed to mark values as "valid" again, when the system switches to it?

[Gertjan van Schijndel]

This issue occurs because the other server does not know that the other server has set the DPE's to invalid, so the values do not pass the lowlevel filter.

I have changed the driver smoothmode to solve this issue (config entry 'drvSmoothMode = 1' or '_Driver.SM' set to 1), so it does not smooth the values generated by a general query, which is executed after a server redundancy switch.

For the future I have asked if the default value for redundant systems can be changed. You can also report this issue, so it gets a higher priority.

[leoknipp]

Hello,

there is no need to report this issue again, it is already noticed in our internal database.

Best Regards
Leopold Knipp
Senior Support Specialist

[n_lev]

Hi Gerjan,

Thanks for your reply. Your explanation sounds logical, I was thinking along those lines too. I wasn't aware of the driver feature you're proposing to use, I'll give it a try first thing tomorrow morning. Thanks a lot for the pointer.

Leopold, does that mean it's a known issue and will be fixed in a new WinCC OA version?

[n_lev]

Happy to report that setting the driver .SM dpe fixed the problem.

Setting the parameter in the config file didn't work though. Not sure why. Anyway, all good now, thanks!

[leoknipp]

Hello,

when the changes will be made is not defined right now. Implementing the necessary changes is more than just setting a dp-element.

Best Regards
Leopold Knipp
Senior Support Specialist