Redundancy issues

[k_r_mehta]

Hello,

I am using Zenon V6.22 SP0. I have created a project with a Primary Server (PC1) and a Standby Server (PC2). The project has one archive trend defined with a few variables.

I transferred all the runtime files to PC2 using the "Project Remote Transport -> Transport all runtime files" context menu. I started the Runtime on PC1 and then on PC2 using "Project Remote Transport -> Start Runtime".

A few minutes later, I disconnect the Ethernet cable from PC1 so that it loses connectivity to the PLC as well as to PC2. I notice that PC2 switches over to its own data acquisition method and starts retreiving data from the PLC. It also gets data for the archive trend variables.

A few minutes later, I connect the Ethernet cable back into PC1. At this point, PC1 becomes the primary server and PC2 switches back to PC1 for its data source. But now when I look at the archive trend data on PC2, it seems that PC2 has lost its data for the time when the Ethernet cable was unplugged from PC1. Similarly the alarms which were showing up on PC2 earlier (alarms which came active when PC1 Ethernet was disconnected), those alarms also disappear from PC2.

Shouldn't PC1 first update its database (alarms and archive trends) from PC2 before becoming the active server? What am I doing wrong? Any help will be greatly appreciated.

[wolfgangm]

Hello,

the situation described is a crucial one.
Basically the whole system behaves as expected.

To get the result you would have expected you would need to shut down the zenon Project on PC1 before plugging the cable back into PC1. After the restart of the zenon project it recognizes that there is a running SERVER and starts up as STANDBY server. In doing so it collects the missing files and only after that switches itself back to SERVER.

Why:
In your scenario, just unplugging the ethernet cable, both PC´s are SERVER. You can see it if you display the system driver variables. If in this situation the connection is reestablished both PC´s see itself as the SERVER ( remember on startup the server starts as standby an switch back later ). To overcome this deadlock it is defined that the original defined SERVER is right, which is in your scenario the PC which had no process connection. That is why in this case the file alignment ran in the "wrong" direction.

This is more or less a undefined situation, how should the system know which PC had process communication and which not.

ATTENTION:
If there is a redundancy switch and both runtimes are running always check what role they think they are in, currently. If it is SERVER - SERVER you MUST check which of both had the process connection, in order to decide which one to shut down before recreation of the connection.

[k_r_mehta]

Thank you very much for the detailed answer. I have verified what you said.

But this basic design is problematic in a plant environment. If the primary server loses its Ethernet connectivity momentarily and then regains it automatically (this happens frequently with Windows due to driver issues or due to an issue with the cable or the switch), in this scenario the primary server overwrites the standby server's data (archived trends and alarms). So perfectly good data is lost.

Isn't it possible to check the date/time stamp of the data and its validity to decide which server (primary or standby) has the latest data and merge it onto the primary server? Or some other scheme which ensures that good available data is not overwritten with invalid data.

Regards.

[wolfgangm]

Hello,

for all to know from zenon 7.00 SP0 on the zenon Redundancy Management Tool (zenon_redman.exe) is available which addresses exactly this issue.

For more information check out the online help (F1), see chapter:
Manual -> Network -> Redundancy -> Redundancy Management Tool

[wrs402]

Hi,
even this issue is some years old - is it possible to stop Zenon RT automatically when the process network interface is in state "unplugged cable" ? So it could prevent that it keeps in master mode even when it cannot get any data and have to be started manually when network is okay ?
How could the network interface status be checked by Zenon RT?

[markclemens]

Hi wrs402,

The reduncancy management tool, as mentioned by wolfgangm, allows you to do exactly that. Also it will automatically restart the runtime, when the network link is up again.





Best regards,
mark