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Asked:18 Oct 2025

WinCC Unified Systems: Multiplex Including Session-local index

I am working on a WinCC Unified project and encountered an issue with a multiplex tag derived from a PLC data type array.
Here is a brief overview of my setup and the problem I am facing:
1.  I created a session-local integer to set the index of the WinCC
Unified multiplex tag.
2.  When attempting this, I receive the error: "Local session tags are not
allowed to be used as index tags for address multiplexing."
3.  My goal is to use this multiplex index tag as a client tag.
The multiplex tag is used as a User-Defined Type (UDT) for my motor pop-up. It functions correctly when only one client is connected. However, if a second client opens the same pop-up with a different index value, the values displayed for the first client change to reflect the second client's index values.
Could you please advise if there is a way to set a multiplex tag index as a client tag to avoid this issue? Any guidance or alternative approaches to achieve the desired functionality would be greatly appreciated.
Thank you for your assistance.

#sosflex #WinCC Unified Systems #siemens
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Answers(1)
Siemens Test Added an Answer on18 Oct 2025

WinCC Unified Multiplex Tag Indexing Issue

You're encountering a known limitation in WinCC Unified regarding multiplex tag indexing and client interactions. The error "Local session tags are not allowed to be used as index tags for address multiplexing" is accurate – session-local tags cannot directly drive a multiplexed address.

The behavior you observe with client values changing is due to the multiplex tag's shared nature. When multiple clients access the same multiplex tag with different indices, they effectively overwrite each other's context.

Here's a recommended approach:

  1. Use a Global Tag for the Index: Instead of a session-local tag, utilize a global tag to store the multiplex index. This ensures a single, consistent index value is used regardless of the client.
  2. Client-Specific Data: If each client truly needs independent data, consider duplicating the UDT structure for each client or designing a data model where each client has its own set of tags.
  3. Indirect Addressing (Advanced): Explore indirect addressing within the PLC to manage the data specifically for each client, potentially using a tag in the PLC to map client IDs to data blocks.

For detailed information and support articles, please refer to the SiePortal knowledge base: Siemens Support. Specifically search for "WinCC Unified Multiplex Tag Client".

Disclaimer: Please verify compatibility and thoroughly test any solutions in a non-production environment before implementation.

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