Comparing Online and Offline CFC Configurations Using Version Cross Manager (VXM) in SIMATIC PCS 7

To compare online and offline CFC configurations using Version Cross Manager (VXM) in SIMATIC PCS 7, you cannot read directly from the S7-400 CPU because controllers store only compiled machine code, not graphical CFC charts. Instead, execute the two-step re-read workflow: (1) In SIMATIC Manager, perform 'Read Back Charts' (Plan -> Rücklesen) to create an offline chart container populated with the actual runtime parameter values from the PLC; and (2) Launch Version Cross Manager (VXM), select your original engineering project as Source 1, select the read-back project (or last-known archive) as Source 2, and click 'Compare'. VXM will perform a graphical and structural XML comparison, highlighting added/deleted blocks in green/red and parameter deviations in yellow.

Executive Summary

During maintenance, commissioning audits, and troubleshooting on SIMATIC PCS 7 systems, plant engineers frequently need to determine whether parameters, setpoints, or block configurations in a live running controller diverge from the offline engineering database. Faced with the requirement to 'check online vs. offline changes', engineers look to the Version Cross Manager (VXM) optional package. However, users often encounter confusion when searching for an 'Online CPU' browsing button inside VXM. Because S7-400 CPUs do not host graphical CFC database records in load memory, VXM relies on a structured Read Back (Rücklesen) and project snapshot methodology to execute comprehensive online-to-offline delta comparisons.

Architecture and Fundamental Mechanics

The comparison mechanics of VXM and the memory architecture of PCS 7 controllers dictate the comparison workflow:

The Nature of S7-400 Controller Memory: When CFC charts compile, the compiler generates machine code: Function Blocks (FBs), System Data Blocks (SDBs), and Instance Data Blocks (DBs). The CPU's RAM and flash memory contain raw operational bits and numbers; they possess no knowledge of graphical chart topologies, sheet layouts, technological hierarchies, or symbolic block names. Consequently, no software can upload a native CFC chart directly from an S7-400 CPU.
The Role of 'Read Back Charts' (Rücklesen): SIMATIC Manager provides the 'Read Back' utility (Chart -> Read Back). This command queries the running CPU's DB memory addresses, extracts operator-entered values (setpoints, tuning constants, alarm limits flagged with S7_m_c or designated read-back attributes), and writes them back into the offline CFC database. This creates an exact offline mirror of the online state.
Version Cross Manager (VXM) Comparison Engine: VXM (licensed under MLFB 6ES7658-1CX58-2YB5 or applicable package) operates by parsing two structured STEP 7 project databases. It decomposes CFC charts, SFC state machines, hardware configurations, and symbol tables into standardized XML trees. When comparing:
- Structural Changes: Identifies blocks, signals, or charts that exist in one project but not the other (color-coded green/red).
- Topological Interconnections: Identifies block wiring differences (color-coded blue).
- Parameter Deviations: Pinpoints value discrepancies between the offline engineering baseline and online read-back parameters (color-coded yellow).
Change Tracking via Version Trail: When paired with SIMATIC Version Trail (VT), engineers can automatically archive every downloaded project state, allowing VXM to compare the current offline project against the exact historical snapshot downloaded to the AS.

Step-by-Step Implementation Procedure

Execute an Online-vs-Offline comparison using the following systematic procedure:

Phase 1: Capture the Online State via Read Back

In SIMATIC Manager, save and archive your current offline project to preserve the engineering baseline (File -> Archive).
Create a duplicate copy of the project (e.g., Project_Online_State).
Open the duplicate project. Select the AS Chart folder (Charts).
Select Chart -> Read Back (Plan -> Rücklesen).
In the Read Back dialog, select the scope: OCM-capable parameters (or All parameters).
Click OK. SIMATIC Manager uploads live runtime values from the CPU and overwrites the block parameters in the duplicate project.

Phase 2: Perform the Comparison in Version Cross Manager (VXM)

Launch Version Cross Manager from SIMATIC Manager (Options -> Version Cross Manager).
In the VXM window, configure the two comparison paths:
- Left Pane (Source 1): Browse to the original engineering project (Project_Offline_Baseline).
- Right Pane (Source 2): Browse to the read-back project (Project_Online_State).
Select the object scope to compare: check CFC/SFC Charts and Hardware Configuration.
Click the Compare button.
VXM parses both project databases and presents a hierarchical delta tree:
- Expand the tree to view specific charts (e.g., Reactor/CFC_Level).
- Yellow highlights indicate parameter deviations (e.g., PID Gain altered online from 1.5 to 2.2).
- Blue highlights indicate modified interconnections.
Export the comparison report to PDF or text format for audit compliance.

Verification

Verify comparison accuracy and project reconciliation:

Parameter Audit: Select a yellow parameter discrepancy flagged by VXM. Open the actual CFC chart online. Confirm that the parameter value in the running CPU matches the value reported by VXM in Source 2.
Report Generation: Click Print / Export in VXM. Verify that the generated audit document cleanly enumerates all changed tags, timestamps, and parameter delta values.
Reconciliation Decision: If the online tuning changes are approved, execute Read Back on the primary project to align the engineering baseline with plant operations.

Limitations and Operational Precautions

VXM comparison requires both projects to be accessible on local or network filesystems; it cannot connect directly to an S7-400 IP address without prior read-back. Furthermore, VXM requires dedicated engineering software licensing. Always confirm license entitlements and order numbers (MLFBs) with an authorized Siemens distributor or the Siemens Industry Mall before deployment.

Engineer FAQ

Why can't VXM connect directly to the PLC online?

The S7-400 CPU does not store graphical CFC charts or symbolic plant hierarchies in its memory. It only holds compiled machine code blocks, requiring the 'Read Back' command to map online data back into a readable chart structure.

What is the difference between VXM and the standard STEP 7 'Compare Blocks' tool?

The standard STEP 7 tool only compares compiled machine code (DB/FB bytecode) without showing which CFC block or pin changed. VXM compares graphical CFC charts, interconnections, hardware configurations, and symbolic tags with full visual delta reporting.

What parameters are uploaded during 'Read Back'?

By default, parameters marked with the S7_m_c := 'true' attribute (Operator Control and Monitoring) are uploaded. Selecting 'All parameters' uploads all writeable inputs and outputs.