Investigate Cannot Write tagname unknown in PCS 7 V9.0 SP1

Investigate the actual client action and tag reference before using full OS compilation with memory reset to address a Cannot Write tagname unknown message. Siemens documents memory-reset cleanup after deleting an AS or S7 program, but that does not establish the root cause of this reported PCS 7 V9.0 SP1 fault. The proposed redundant-server repair lacks a confirmed result and includes a master-tag spelling discrepancy requiring review.

Separate the Symptom from a Proven Cause

The reported installation has two redundant OS servers and five clients. Unknown-tag write messages occur mainly on one client; APDiag did not provide a matching entry. A reply proposed full OS compilation with memory reset, but no successful final result was reported.

Do not infer that every such message results from manually added tags. First record the exact message, affected workstation, displayed picture and operation. Compare the tag references and installed project state on affected and unaffected stations.

What Memory Reset Establishes

For V9.0 SP1, Siemens states that tags and connections associated with a deleted AS or S7 program are removed only after full compilation with memory reset. That is a documented cleanup condition, not a verified general cure for unknown-tag write messages. Operator Station, section 19.6.

The proposed repair also warned about manually added tags inside automatically created connections. Their behavior in this exact project has not been independently established. Inventory and compare these customizations in a project copy before relying on regeneration.

Documented Compilation Controls

The V9.0 SP1 software-update manual documents these compilation controls in its update context:

Select the object in SIMATIC Manager and use PLC > Compile and download objects.
Expand the tree; select the intended objects in the Compile column.
Check Operating State and select the OS; click Edit.
Continue through the Areas and Network Connections pages.
Under Scope, select Entire OS and With memory reset.
Click Apply, wait for the download dialog, and review individual object settings.
Start the selected operation and inspect the compilation log.

This is a documented workflow within the manual's prerequisites. For fault investigation, prepare and inspect a backed-up engineering copy first; do not treat the combined compile/download dialog as authorization to load production stations. Software update, section 8.10.3.

Protect the Redundant Runtime

A source-reported staged-server approach is not established here as a complete approved recovery sequence. Define the actual download type, server availability, client access, rollback and synchronization requirements before production work.

The quoted master-tag spelling conflicts with Siemens' WinCC V7.4 SP1 migration example, which uses @RM_Master. That example is scoped to redundant-system migration; it does not by itself validate this proposed repair sequence. Do not apply an unverified master-switch command. WinCC manual, section 5.9.

Verification

Compare the engineering copy before and after compilation: required connections, generated tags and manually configured references must remain accounted for. Resolve missing custom items before loading.

For an subsequently approved deployment, observe the original client action and message recurrence, check the other clients and both OS servers, and confirm redundancy behavior against the site's acceptance plan. A clean compilation log alone does not prove the unknown-tag problem is resolved. No deployment or lab test is claimed.

Limitations and Operational Precautions

The triggering action, exact tag reference and successful final remedy are absent. Memory reset remains a proposed intervention for this symptom. Its consequences for custom data and the staged redundant-server procedure require review.

Retain a restorable ES/OS backup and a rollback plan. Keep offline compilation, production download and master switching distinct. Do not accept a blanket claim that the procedure has no operational effects.

Engineer FAQ

Is memory reset a confirmed cure here? No; the reported proposal lacks a confirmed outcome.

Does the documented deletion cleanup apply to every missing tag? No; its stated condition is deletion of an AS or S7 program.

Can the quoted master tag be copied literally? Its spelling conflicts with the opened manufacturer example and requires review.

Technical References

Related Tags

OS Redundancy

Tag Management

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Investigate Cannot Write tagname unknown in PCS 7 V9.0 SP1