Resolving Insufficient Memory Errors in SIMATIC PCS 7 Version Cross Manager (VXM)

In SIMATIC PCS 7 engineering environments, 'Insufficient main memory' or 'Your computer is low on memory' errors encountered during project comparisons in Version Cross Manager (VXM) occur when the 32-bit comparison engine exhausts its 2 GB virtual address space while constructing in-memory XML document object models (DOM) for massive multiprojects. Resolving this issue requires scoping comparisons to individual automation systems or chart folders rather than comparing entire multiprojects at once, reorganizing project databases to clear fragmented records, and ensuring the latest VXM updates are installed.

VXM Memory Architecture and 32-Bit Process Limits

SIMATIC Version Cross Manager (VXM) is an engineering tool used to perform detailed graphic and tabular difference comparisons between two versions of a PCS 7 project, automation system, chart folder, or hardware configuration. VXM parses the underlying Step 7 database objects—including hardware configurations, symbolic tables, CFC charts, SFC types, and block parameters—and converts them into structured hierarchical object trees in memory.

Because VXM operates as a 32-bit Windows process (vxm.exe), it is bound by the standard 2 GB virtual address space allocation limit per process, regardless of whether the engineering station workstation possesses 16 GB, 32 GB, or 64 GB of physical RAM. In large-scale PCS 7 multiprojects containing tens of thousands of process objects, millions of interconnection parameters, and multiple AS/OS projects, VXM's in-memory XML/DOM tree construction rapidly saturates available virtual memory addresses, resulting in memory allocation failure exceptions.

Scoping and Filtering Comparisons to Reduce Memory Footprint

To successfully execute comparisons without triggering memory exhaustion, engineers must reduce the comparison scope:

Avoid Multiproject-to-Multiproject Comparison: Never select the root Multiproject node for comparison if the project contains more than two automation stations or extensive OS server data.
Compare at the AS Project Level: Open VXM and select a single AS project (e.g., AS01) from Backup Date 1 and compare it against AS01 from Backup Date 2.
Compare Individual Chart Folders: If an individual AS project is exceptionally large (e.g., containing over 500 CFC charts), select only the Charts folder or individual technological subfolders.
Exclude Hardware Configuration: If hardware changes are known to be absent, compare only the software block containers to eliminate hardware catalog XML overhead.
Close Concurrent Engineering Applications: Close SIMATIC Manager, WinCC Explorer, CFC editors, and S7-PLCSIM before launching VXM to maximize continuous virtual address space available to the operating system.

Database Reorganization and Project Health Maintenance

Database fragmentation and orphaned records in older project backups significantly increase memory consumption in VXM:

Open SIMATIC Manager: Open each of the two project backups independently.
Execute Slow Reorganization:
- Navigate to File > Reorganize... (or select File > Save As... and check the box With reorganization (slow)).
- The reorganization utility parses the Step 7 database, eliminates deleted object fragments, rebuilds internal B-tree indices, and optimizes data tables.
Perform Cross-Reference Rebuild: In SIMATIC Manager, execute Options > Reference Data > Generate to ensure cross-reference tables are consistent.
Re-attempt VXM Comparison: Launch VXM and point the source and target selectors to the freshly reorganized project files.

Memory Optimization Strategy Matrix for VXM

The following table details recommended operational strategies to prevent VXM memory exhaustion:

Comparison ScopeProject ComplexityRecommended ActionRisk of Memory ErrorEntire Multiproject (>3 AS, Redundant OS)High (>5,000 POs)Do not execute. Split comparison into individual AS stations.Extreme (Failure imminent)Single AS ProjectMedium (1,000–3,000 POs)Execute 'Save As with reorganization (slow)' prior to comparison.Low to ModerateSingle Chart Folder (CFC/SFC)Focused ScopeDirect comparison in VXM; highly reliable.Very LowHardware Configuration OnlyHW Config changesOpen HW Config comparison directly from Station menu.Very LowPlant Hierarchy Sub-AreaHierarchicalSelect specific branch in Plant View comparison.Low

Note: Siemens Service Packs and Updates for Version Cross Manager (such as VXM V8.2 Upd 1 or V9.0 SP1) include critical memory management and leak fixes. Always verify that the installed VXM build matches the latest released patch level for your PCS 7 version.

Verification

Verify that VXM comparisons complete successfully:
1. Launch VXM with scoped project components and initiate the comparison run.
2. Open Windows Task Manager and monitor the vxm.exe process under the Details tab; verify that private memory usage remains stable below 1.5 GB.
3. Verify that the VXM difference tree renders completely, showing color-coded deviations (added, deleted, modified objects).
4. Export the comparison report to an XML or PDF file to confirm complete data serialization.

Limitations and Operational Precautions

Observe these essential limitations when operating VXM:
- VXM comparison reports only show differences in configuration data; they do not compare runtime historical process values or controller memory contents.
- Do not modify project files while VXM is reading the database, as file lock collisions will corrupt the comparison tree.
- Ensure both compared projects were engineered with compatible library versions; comparing projects across mismatched major APL releases will generate thousands of false-positive block version differences.

Engineer FAQ

Why does VXM run out of memory when my PC has 32 GB of RAM?

VXM is a 32-bit executable and is architecturally restricted to a 2 GB virtual address space by the Windows operating system, regardless of total physical RAM.

Can I use Version Trail alongside Version Cross Manager?

Yes. SIMATIC Version Trail manages project archiving and version history, while VXM is the comparison engine invoked by Version Trail to inspect differences between historical versions.

What does 'Save As with reorganization (slow)' do to help VXM?

It eliminates deleted records, removes internal database fragmentation, and compacts tables in the Step 7 database, significantly reducing the memory required by VXM to parse project objects.