Preserving Delta Download Capability (Changes-Only Download) When Unpacking PCS 7 Multiprojects

Preserving the ability to perform a changes-only delta download ('Laden von Änderungen') to an S7-400 or CPU 410-5H controller without stopping the CPU after unpacking (dearchiving) a PCS 7 multiproject requires binary timestamp congruence between the offline chart database and the online CPU program. Delta download capability is governed by internal compile and download timestamps ('Ladestempel') stored in the system data blocks. As long as the archive was generated immediately after the last successful download—without subsequent online test mode edits, block re-imports, or offline chart recompilations—dearchiving the project retains 100% delta download capability. To safeguard this, never perform an unverified 'Entire Program' compile or re-import block types before establishing online synchronization.

Executive Summary

In 24/7 continuous process industries running SIMATIC PCS 7, unprompted controller shutdowns ('Urladen' / Complete Download in STOP) represent catastrophic downtime risks. A critical operational question arises when retrieving archived multiprojects: Does dearchiving a backup risk invalidating the delta download handshake, forcing an AS stop? Under PCS 7 architecture, archiving (File > Archive) and dearchiving (File > Retrieve) use lossless PKZip compression that preserves all internal database tables, compilation logs, and download stamps intact. Loss of delta download capability occurs not from the extraction process itself, but from improper post-retrieval workflows, such as updating Master Data Libraries, running unconstrained full recompilations, or saving charts after online test sessions prior to backup.

Architecture and Fundamental Mechanics

The PCS 7 CFC and STEP 7 engineering environment uses a cryptographic and timestamp handshake to enforce synchronization between the offline database and the runtime CPU:

The Download Stamp Mechanism (Ladestempel):
* When CFC compiles a program (Chart > Compile > Charts as Program), it generates executable machine code organized into System Data Blocks (SDBs), function blocks (FBs), and data blocks (DBs).
* The compiler generates a unique, 64-bit compilation timestamp and program signature stored within System Data Block SDB 200 and internal DB headers.
* When a download is performed, this signature is transferred to the CPU's load memory and work memory.
Handshake Verification During Download:
* When an engineer selects PLC > Download > Scope: Changes only, STEP 7 queries the online CPU for the active timestamp in memory.
* If the timestamp in the offline project matches the timestamp in the online CPU, STEP 7 calculates the delta: it identifies which DBs, FBs, and execution sequences changed, downloads only the modified blocks into free memory, atomically updates pointers during cyclic interrupt boundaries (OB3x), and releases old memory blocks—all while the CPU remains in RUN.
* If the timestamps differ (due to an intermediate compile, block interface change, or chart renumbering), STEP 7 reports that delta download is impossible and requires a full download in STOP.
Integrity During Archiving and Retrieving:
* STEP 7's internal archiving utility compresses the entire project folder structure, including the hidden s7bin, baustein, and CFC database files (amdbase.db, chart.db).
* No metadata, compile timestamps, or load stamps are altered or stripped during archiving or retrieval. Therefore, a dearchived project is mathematically and functionally identical to the state when it was archived.
Triggers for Delta Download Loss:
* CFC Test Mode / Online Monitoring: Opening test mode or dynamic value monitoring does not change timestamps unless an engineer edits a parameter value marked as 'Compile-relevant' or saves chart structural changes.
* Block Type Interface Changes: Updating a block type in the Master Data Library that alters the block interface (adding/removing inputs/outputs or changing data types) invalidates the delta download capability for that block's instances.
* Unsynchronized Compilations: Running a full compile ('Entire program') generates a brand-new compilation timestamp, breaking the handshake with the CPU.

Step-by-Step Implementation Procedure

Follow these best-practice procedures to unpack a PCS 7 multiproject while guaranteeing delta download capability:

Establish a Strict Baseline Archive Routine:
* Immediately following any successful download to the physical or virtual controller, perform File > Archive in SIMATIC Manager.
* Store the archive with an explicit naming convention including date, time, and download state (e.g., Plant_MP_V903_20261014_PostDownload.zip).
Retrieve (Unpack) the Multiproject Safely:
* Open SIMATIC Manager and select File > Retrieve....
* Select the archive file and choose a local, uncompressed destination directory (e.g., D:\PCS7_Projects\).
* When prompted, allow SIMATIC Manager to open the retrieved multiproject.
Verify Timestamp and Online Congruence Before Any Edits:
* Do NOT compile charts or modify hardware configuration immediately after retrieval.
* In SIMATIC Manager, select the AS station's S7 Program folder.
* Execute PLC > Compare Blocks... against the Online CPU. Verify that all standard and safety blocks report matching timestamps and code checksums.
* In CFC, select Target system > Compare. Verify that the offline chart database matches the online controller.
Execute Engineering Changes Safely:
* Make the required logic modifications within the CFC charts.
* Do NOT update block types from external libraries unless specifically planning for interface upgrades.
* Do NOT change CPU cycle execution parameters or memory allocations in HW Config unless required.
Compile Only Changes:
* Open CFC and select Chart > Compile > Charts as Program.
* Under Scope, select Changes only (ensure 'Entire program' is NOT selected).
* Ensure 'Generate module drivers' is selected if hardware signals changed.
* Click OK and review the compilation log for zero fatal errors.
Perform Changes-Only Download in RUN:
* Select PLC > Download in CFC.
* In the Download dialog, verify that the radio button Changes only is active and selectable.
* Confirm that the download completes while the CPU remains in RUN mode.

Verification

Verify delta capability by opening the Download dialog in CFC. Confirm that 'Changes only' is selectable and not grayed out. Initiate the download; confirm that the progress bar displays delta block transfers (OBs, DBs) and that SIMATIC Manager indicates the CPU remained continuously in RUN throughout the transfer. Inspect the AS diagnostic buffer (PLC > Module Information > Diagnostic Buffer); confirm that no 'STOP request' or 'Timestamp mismatch' events were recorded.

Limitations and Operational Precautions

Delta download will be permanently blocked (requiring a CPU STOP) if you modify the block interface (inputs/outputs) of an FB that is instantiated in active charts, if you delete an OB or runtime group containing active logic, or if you exceed the available free work memory in the CPU. When working with S7 F Systems (fail-safe logic), any change to safety runtime groups requires safety password entry and a dedicated F-program delta compile.

Engineer FAQ

Does archiving and retrieving a project in SIMATIC Manager alter any compile stamps?

No. The archiving and dearchiving process is completely lossless; all compile stamps, block timestamps, and SDB signatures remain byte-identical.

Why did CFC ask for a complete download after I only opened test mode?

Opening test mode alone does not invalidate delta download. However, if an online value change was saved back into the offline chart, or if an engineer accidentally clicked 'Save' on a chart with pending structural changes, the compile stamp was updated.

What should I do if 'Changes only' is grayed out?

If 'Changes only' is grayed out, the offline compile timestamp does not match the online CPU. You can attempt to restore a prior archive taken immediately after the last download, or you must schedule a plant downtime window for a complete download in STOP.

Can I restore delta download capability without stopping the controller?

No. Once the handshake timestamp is lost between the active offline project and the running CPU, STEP 7 cannot guarantee memory safety and will refuse delta downloads until a full download in STOP synchronizes them.