C135 PCS 7 (PCS7)
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Understanding OS Delta Download Quantity Limits and Handling Delta Overflows in SIMATIC PCS 7

Understanding OS Delta Download Quantity Limits and Handling Delta Overflows in SIMATIC PCS 7

In SIMATIC PCS 7, the maximum quantity limits for 'Download to target system -> Changes only' (OS Delta Download) are hardcoded internal architectural thresholds within WinCC and cannot be adjusted or expanded through configuration settings, registry keys, or configuration files. When the volume of modified process pictures, Tag Logging tags, or alarm definitions exceeds these internal limits, the system issues a warning and disables or rejects the delta transfer. The only valid and supported engineering response is to perform an 'Entire OS' download, which in redundant server configurations can be executed sequentially online without causing process downtime.

Table of Contents

Architectural Mechanism of OS Delta Downloads

SIMATIC PCS 7 enables engineers to update operator station configurations online through the 'Changes only' (Delta Download) mechanism without halting active process visualization. When an engineer initiates a Delta Download from SIMATIC Manager, the system relies on an underlying differential comparison engine:
1. Change Tracking in Engineering System (ES): When CFC charts, SFCs, or WinCC graphics are compiled, SIMATIC Manager records modified objects, deleted variables, and newly created tags in temporary delta journal tables within the project database (CS_DB).
2. Target Comparison: The download service queries the target OS Runtime database (RT_DB), calculates the precise binary difference, and streams delta packages across the Terminal Bus.
3. Online Database Swapping: WinCC Runtime services (such as CCAlgRtServer, CCTlgServer, and PDLRT) reload modified graphics pages, dynamically register new tag structures in memory, and update SQL Server database schemas without interrupting active communication with the automation controllers.

Why Delta Download Quantity Limits Cannot Be Adjusted

When making extensive modifications across hundreds of process pictures, bulk-generating new alarm blocks, or updating large technological libraries, engineers often encounter the WinCC notification:
'The number of changes is greater than the limit value. A complete upload/download is recommended.'

Engineers frequently investigate whether configuration files (such as WinCC.ini, OS.ini, or Windows Registry values) can expand these delta quantity thresholds. Siemens explicitly design-locks these limits; they cannot be modified.

Technical Rationale Behind the Hard Limits

  • SQL Server Transaction Buffer Constraints: During a delta download, modified alarm schemas and tag logging structures must be staged in transient memory journals. Exceeding buffer limits risks uncommitted transaction aborts, leading to corrupt SQL master tables.
  • Redundancy Synchronization Bandwidth: In redundant OS architectures, the active master server must replicate every delta transaction to the standby server via Redundancy (CCRedSync.exe). Excessively large delta packages cause synchronization queues to overflow, risking server failovers or split-brain conditions.
  • Graphical Runtime Latency: Reloading hundreds of dynamic picture elements on an active operator station can freeze the graphics rendering thread (PDLRT.exe), causing temporary operator blindness.

Workflow for Handling Delta Quantity Overflows in Redundant Systems

When delta quantity limits are reached, engineers must transition to a full download. In redundant architectures, a complete download is executed without process interruption by following this sequential procedure:

Step 1: Compile the OS Completely

  1. In SIMATIC Manager, right-click the OS Server object and select Compile.
  2. Select Entire OS (with memory reset if database inconsistencies are suspected).
  3. Ensure compilation completes with zero errors.

Step 2: Download to Standby Server First

  1. Identify the currently active Server (Master) and Standby Server via WinCC Explorer status or the @RM_MASTER internal tag.
  2. In SIMATIC Manager, initiate Download to target system -> Entire OS targeting the Standby Server.
  3. Deactivate WinCC Runtime on the Standby Server when prompted.
  4. Allow the full database and picture files to transfer completely.

Step 3: Activate Standby Server and Verify Synchronization

  1. Activate WinCC Runtime on the Standby Server.
  2. Monitor Redundancy Synchronization in the status bar or using the WinCC Redundancy Control window.
  3. Wait until historical tag archives, alarm journals, and internal statuses have synchronized completely (Redundancy state indicates 'Synchronized').

Step 4: Download to Master Server

  1. With the Standby Server actively servicing operator clients, initiate Download to target system -> Entire OS targeting the primary Master Server.
  2. Deactivate Runtime on the primary server, complete the download, and reactivate Runtime.
  3. Wait for reciprocal synchronization to finish.

Delta Download vs Full Download Operational Comparison

The following table summarizes the capabilities, operational constraints, and boundary conditions of OS download modes:

Operational ParameterOS Delta Download (Changes Only)Entire OS Download (Full)Runtime InterruptionZero downtime (Continuous Runtime)Requires Runtime deactivation on target nodeRedundant Server ProcedureSimultaneous online background updateSequential download (Standby first, then Master)Maximum Change ThresholdEnforced internal limit (Pictures, Tags, Alarms)Unlimited (Complete database overwrite)User Threshold AdjustmentNot possible (Hardcoded system protection)Not applicableModifiable ElementsGraphics, Tag properties, Alarm textsComplete project schema, HW Config, Script enginesTypical Execution Duration30 seconds to 3 minutes5 to 15 minutes per server node

Always perform sequential downloads during low plant operational activity to ensure continuous monitoring.

Verification

To verify successful project updates following a delta overflow resolution:
1. On both redundant OS Servers, verify that the WinCC status bar shows 'Runtime Active' with no DEMO or licensing warnings.
2. Verify that the internal tag @RM_MASTER is correctly assigned on the active node and @RM_SERVER_STATE reports normal operation.
3. In WinCC Explorer, open Diagnostics -> Redundancy and confirm that archive synchronization has completed with zero error logs in Redundancy.txt.
4. Open newly added process pictures on an OS Client station and verify that new dynamic objects, faceplates, and trend curves update with live AS process values.
5. In SIMATIC Manager, initiate a test delta compile of a minor comment change to verify that the delta compilation mechanism is restored.

Limitations and Operational Precautions

  • Never initiate an Entire OS download simultaneously to both servers of a redundant pair, as this causes total loss of operator visualization.
  • When executing an OS Delta Download, ensure that no other engineer is modifying the project concurrently on another ES client.
  • Modifying basic system components in OS Project Editor (such as screen resolution or layout templates) invalidates delta download capability, requiring an Entire OS download.
  • Avoid making bulk tag logging modifications during critical plant maneuvers; schedule major updates during planned maintenance windows.

Engineer FAQ

Can I bypass the delta quantity warning by downloading multiple smaller batches?

Yes. If you make smaller incremental changes in CFC or WinCC and compile/download changes after each small batch, you can stay below the delta threshold and avoid a full download.

What triggers an automatic invalidation of the delta download option?

Actions such as deleting an OS connection in NetPro, changing the WinCC project type, modifying OS Project Editor basic settings, or running an OS compile with memory reset permanently disable delta downloads until the next full compile and download.

Does an Entire OS download to an OS Server affect running AS controllers?

No. OS downloads only affect the PC-based supervisory stations. The S7-400 or CPU 410-5H controllers continue running their automation logic uninterrupted.

Technical References

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Related Tags

  • OS_Delta_Download
  • WinCC_Redundancy
  • Download_Changes
  • SIMATIC_PCS7
  • OS_Server
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