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DCOM - Have synchronous communication, need asynchronous

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21 Mar 2016 07:45 #3941 by support

Replied by support on topic DCOM - Have synchronous communication, need asynchronous

I do not have explanation for why the original 6 computers work and the new ones don't. There are so many factors and the complexity of DCOM security is so enormous that it is many times difficult to set it up correctly for an expert when present on site, let alone via remote support channel like this.

OPC itself has parts that can work without the reverse direction communication - e.g. the browsing (as you have already found out), but also the synchronous reads & writes, "get properties" and a handful of other functionalities. Parts that won't ever work without the reverse direction communication are the async reads/writes, and subscriptions.

The OPC client may be programmed to be "strict" in the sense that when it encounters a problem setting up the callbacks, it would not allow you further. Or, it can be more forgiving, and allow at least the functions that are accessible.

I do not know which OPC client you are using. The forums here are for our product, QuickOPC. While QuickOPC prefers async reads/writes, newer versions of QuickOPC will fall back to sync reads/writes if async is not available. Subscriptions, of course, won't work in this case. If you are using QuickOPC, please state its version & build.

Best regards

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20 Mar 2016 19:33 #3940 by says

DCOM - Have synchronous communication, need asynchronous was created by says

I have one server and 10 computers. Each of the 10 computers has an OPC server running. We recently replaced 4 of the computers with their original, pre-configuration images. I am trying to destablish DCOM communication from the server to the 10 computers.

So I am running an OPC client on the server as, we'll say User1. I have added User1's login details as a user under computer management for the 4 new computers. After configuring security access limits on COM and the OPC server DCOM, I can browse the computer for OPC servers and connect to my OPC server. Everything seems fine.

BUT I only have synchronous communication, meaning the computer cannot connect back to the server. I believe this to be the case because the computer is logged in under User2, and the OPC server is being run as the interactive user - which would also be User2.

For reasons outside of my control, I do not have access to User2's password, and I cannot change how the OPC server is being run. This would imply that asynchronous DCOM communication is not possible - the computer's credentials cannot be validated on the server.

Except for one thing - the other 6 computers all have the exact same configuration, and they DO have full asynchronous DCOM communication. I cannot for the life of me figure out how these computers were configured to behave in this way - this was done 8+ years ago and the people who did it have since left my company or retired.

What am I missing? Is there some way to bypass the asynchronous callback windows authentication?

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