Designing Long-Term Tag Archiving and Direct Process Access for Distributed OS Single Stations in SIMATIC PCS 7

In SIMATIC PCS 7 architectures comprising multiple distributed plant areas (e.g. 6 independent areas each controlled by a dedicated OS Single Station), achieving centralized cross-area process monitoring and long-term historical tag archiving without creating controller communication lag requires deploying SIMATIC Process Historian (PH) alongside SIMATIC Information Server (IS), or structuring the system as a Multi-Client / Distributed Server architecture. Attempting to have multiple OS Single Stations poll each other's PLCs directly overburdens controller communication channels and introduces substantial network latency. The approved solution decouples real-time AS-OS control from long-term reporting via dedicated Terminal Bus streaming.

Architecture Challenges in Distributed OS Single Station Plants

A frequent design dilemma in medium-to-large industrial facilities (such as water treatment plants, tank farms, or pharmaceutical complexes) involves balancing local operational autonomy with centralized management:
- The 6-Area OS Single Station Layout:
* Each of the 6 independent plant areas operates its own S7-400 controller and a dedicated OS Single Station (e.g. IPC647D / IPC627D).
* This guarantees that if one area network or workstation fails, the remaining five areas continue autonomous production without interruption.
- The Central Access Bottleneck:
* Corporate managers, environmental compliance officers, and plant supervisors require enterprise-wide trend analysis, centralized alarm logging, and multi-year data archiving.
* If engineers attempt to configure each OS Single Station to cross-poll tags directly from the other 5 automation stations across the plant network:
* S7-400 CPU communication connection resources are rapidly exhausted.
* Real-time loop control latency increases as controller communication processors (CP 443-1) become saturated with asynchronous WinCC polling requests.

Comparing Centralized Archiving Approaches - Process Historian vs Multi-Server

Siemens PCS 7 engineering standards offer two robust architectural patterns:

Option A: Distributed OS Single Stations with Central Process Historian (Recommended):
- Each OS Single Station retains local autonomy for real-time control and short-term buffering (e.g. 7 to 30 days in local ArchiveManager).
- The Process Historian Ready (PH-Ready) service is installed on all 6 Single Stations.
- PH-Ready automatically streams completed archive segments over the Terminal Bus to a central SIMATIC Process Historian (PH) server.
- A co-located SIMATIC Information Server (IS) provides web-based reporting, dashboards, and Excel export across all 6 areas without placing any polling load on the S7-400 controllers.
Option B: Multi-Client / Distributed Server Architecture:
- The 6 stations are configured as OS Servers (or redundant server pairs).
- Dedicated OS Clients (or WebNavigator clients) in the central control room load Server Packages from all 6 servers simultaneously.
- This provides a unified operator screen where an operator can view and control any unit across the 6 areas seamlessly, while process data continues to flow through optimized server proxies.

Network Segmentation Rules - Preventing Process Image Latency

To ensure that cross-station communication never induces lag in real-time control:
- Strict Separation of Plant Bus and Terminal Bus:
* Each OS Single Station must have two physical network adapters.
* Network Adapter 1 (Plant Bus): Dedicated strictly to S7 Named Connections between the local OS and its respective S7-400 CPU (e.g. 192.168.1.x). No inter-workstation or office traffic is permitted on this subnet.
* Network Adapter 2 (Terminal Bus): Interconnects all 6 Single Stations, the Process Historian, Information Server, and central engineering workstations (e.g. 172.16.10.x).
- Simatic Shell Configuration:
* Simatic Shell must bind strictly to the Terminal Bus adapter to handle inter-station broadcasts, time synchronization, and historian data delivery.

Distributed Single Station vs Centralized Historian Comparison Table

The following table compares architectural configurations for multi-area plants:

Architectural ApproachLocal Autonomy If Network FailsLong-Term Data Archiving ScopeReal-Time AS-OS Communication ImpactCentral Reporting CapabilityDirect Cross-Polling (Single Stations)HighLow (Local hard drives only, 1–3 months)Severe (Exhausts PLC connection resources)Poor (Scattered data, manual exports)Single Stations + Process Historian100% AutonomousHigh (Multi-year, scalable SQL database)Zero (Streaming handled via Terminal Bus)Excellent (Web dashboards, scheduled reports)Distributed Multi-Server ArchitectureHighMedium (Requires distributed archive linking)Minimal (Optimized client-server fanout)High (Unified operator screen across 6 areas)WebNavigator ArchitectureHighLow (Dependent on base server storage)Zero (Web server proxies client requests)Medium (Web-based remote view of local screens)

Verification

To confirm that the distributed architecture operates with zero controller communication lag:
1. In SIMATIC Manager, open the CPU Diagnostic Buffer on each S7-400 station: verify that communication connection usage is under 50% of available resources.
2. In WinCC Explorer on each Single Station, open WinCC Channel Diagnosis: verify that average cycle times for Named Connections remain below 50 ms.
3. Open the Process Historian Management Console: verify that all 6 OS Single Stations are recognized under Plant Structure with status Active / Streaming.
4. Launch SIMATIC Information Server: generate a cross-area historical report combining analog tags from Area 1 and Area 6. Verify that the report compiles in under 5 seconds without triggering CPU communication alarms.

Limitations and Operational Precautions

When deploying distributed architectures:
1. Never Share IP Subnets Across Plant and Terminal Buses: The Plant Bus and Terminal Bus must use distinct, non-routable IP subnets to prevent Windows routing loops and broadcast packet storms.
2. Plant-Wide Time Synchronization: All 6 OS Single Stations and S7-400 controllers must synchronize to a single central master clock (e.g. SICLOCK TC400 via NTP) in UTC to prevent timestamp collisions in the central Process Historian.
3. Local Disk Space Monitoring: Configure automated circular archiving in WinCC Tag Logging on each Single Station to prevent local hard drives from filling up if network communication to the central historian is temporarily severed.

Engineer FAQ

Can an OS Single Station run without a Process Historian?

Yes. Each OS Single Station operates completely autonomously and can store local Tag Logging and Alarm Logging data for weeks or months depending on allocated hard disk space. Process Historian is required only for long-term multi-year archiving and cross-plant reporting.

How many tags can Process Historian archive simultaneously?

Process Historian is scalable to process tens of thousands of archive values per second across dozens of OS servers, backed by Microsoft SQL Server Enterprise edition.

Can an operator on Single Station 1 control a valve on Controller 2?

Not natively, unless cross-AS communication is established in the controllers (via S7 connections using BSEND/BRECV or S7-400H cross-talk), or the stations are converted into a Multi-Client / Distributed Server architecture.

Does PH-Ready consume significant CPU resources on the Single Station?

No. PH-Ready operates as a low-priority background Windows service that transfers completed, closed database archive segments asynchronously, leaving real-time WinCC runtime processes completely unhindered.