PCS 7 CAS Configuration with Multi-Single OS Station Architecture

Siemens PCS 7 Central Archive Server (CAS) is a long-term data archival layer for WinCC/OS process values, alarms, and messages. It is designed to operate as a redundant or non-redundant paired service that receives incoming data from one or more OS Servers. A persistent limitation across PCS 7 V6.x, V7.0, V7.1, V8.0, and V8.1 is that the CAS WinCC archive connector expects an OS Server-class archive source. Standalone "Single Station" OS projects do not expose the multi-client archive interfaces that the CAS connector enumerates, so the CAS pairing logic silently refuses the subscription or fails to populate the long-term archive. This document provides a field-engineering reference for resolving that limitation in a typical Multi-Single-Station plant, with the recommended migration path from CAS to Process Historian (PH) introduced in PCS 7 V8.0.

1. CAS Architectural Constraints

The CAS component is implemented as a WinCC server-side archive with a connector agent that polls OS Servers on a configurable network path. Three constraints govern whether a CAS instance can attach to a given OS station:

1. The OS must publish the WinCC archive via the standard server-client segment (multi-client or server-with-project). Single Stations operate as isolated, self-contained WinCC projects without the multi-client segment, so the archive handshake fails.
2. The OS Server must be reachable on the plant LAN and the archive account credentials must match the CAS service user. Domain trust or local-equivalent accounts are required for unattended archival.
3. CAS must be installed on a separate Windows PC (Server 2003/2008/2012 depending on PCS 7 version). Co-installation with the ES or any OS station is unsupported and is the most common cause of the "0 segments found" condition in the CAS configuration tool.

2. Multi-Single-Station Topology Reference

A Multi-Single-Station configuration is a common brownfield arrangement in which the plant runs multiple autonomous PCS 7 OS Single Stations (each with its own local archive, alarms, and pictures) rather than a centralized OS Server. The Multi-Single-Station pattern is typically chosen when:

• Sub-plants are physically separated and have weak inter-station network reliability.
• Process ownership is decentralized and each cell must operate independently if the plant network fails.
• Brownfield migration from legacy WinCC Single Stations is staged per area.

The reference topology this article addresses is:

2.1 Why the CAS Does Not Pair with a Single Station

The WinCC archive runtime exposes two distinct external interfaces:

• Server archive interface – used by WinCC Clients and by the CAS connector to subscribe to tag and alarm streams.
• Local archive interface – used by a Single Station's own internal trending and reporting, not exposed externally.

Because Single Stations only expose the local interface, the CAS connector cannot enumerate the archive segments. The WinCC Explorer on the CAS side shows "0 segments available" under Computer → Properties → Archive. This is the most reliable diagnostic symptom and is independent of network reachability, Windows user rights, or DCOM configuration.

3. CAS vs. StoragePlus vs. Process Historian Decision Matrix

Three long-term storage paths are available across the PCS 7 V7.x and V8.x line. The selection depends on the OS topology and the target PCS 7 version.

4. Required Architecture Correction: Server-Client Refactor

If the PCS 7 version is locked to V7.1 (or any V6.x–V8.0 line) and a central archive is required, the OS layer must be re-architected into a Server-Client model. The minimum topology for a single CAS deployment is one OS Server, one or more OS Clients, and the CAS on its own hardware. For a 4-cell plant this maps to:

The license delta between 4 × Single Station and 1 × OS Server + 4 × OS Client is typically a net saving for the historian-capable configuration when central long-term storage is required. This trade-off should be presented to the plant owner before the refactor is scoped.

4.1 Server-Client Refactor Procedure (PCS 7 V7.1)

1. Open the project in SIMATIC Manager on the ES and select the OS Single Station object.
2. From the OS context menu choose OS Project Editor → Assign OS Server. Create a new OS Server project and a matching OS Client project per cell.
3. Re-assign each cell's WinCC pictures, archives, and tag subscriptions to the new OS Server. Move the runtime database segments from the Single Station's local path to the OS Server's network share.
4. On the dedicated CAS PC install Windows Server 2003/2008, apply the WinCC CAS installation, then run the CAS Configuration tool. Add the OS Server by name; confirm the archive segments enumerate under Computer → Properties → Archive.
5. Configure archive tags, compression, and retention per the project's regulatory retention schedule. The default 30-day OS Server archive is separate from the long-term CAS retention; the CAS retention is the controlled record.
6. Validate with a 24-hour burn-in. Compare the CAS segment count against the OS Server segment count in WinCC Explorer's Archive view; the values must match within one cycle.

5. Process Historian Migration Path (V7.1 → V8.0+)

When a Multi-Single-Station topology is fixed (e.g. decentralized process ownership) and the customer needs a long-term archive, the recommended resolution is to upgrade PCS 7 to V8.0 or higher and deploy the Process Historian. PH is the documented CAS replacement and the only path that supports direct Single Station ingest.

5.1 Prerequisites for the V7.1 → V8.0 Migration

• Authoritative engineering time on the ES – at least 8 to 16 hours per V7.1 project depending on library customization depth.
• Re-installation of the OS Single Stations with the V8.0 Single Station image. The V7.1 image is not upgradeable in place; a clean install is required.
• Separate hardware for the PH (Windows Server 2008 R2 or 2012 R2 supported for V8.0; 2012 R2 / 2016 / 2019 for V8.2+). Co-location with CAS or OS is not supported.
• Re-validation of all AS program blocks against the V8.0 APL library. The PCS 7 V8.0 master data library contains a renamed and re-versioned APL that requires block-for-block recompilation in the S7 program.
• Re-export of the OS pictures to the V8.0 WinCC graphics designer. Several V7.1 faceplate objects are not binary-compatible.

5.2 Process Historian Ingest from Single Stations

Once the OS Single Stations are re-imaged at V8.0 or higher, the PH connector on each Single Station is configured to forward process values and alarms to the PH. The configuration is in the WinCC Explorer of the Single Station under PH Server → Connection. The PH server is identified by hostname and the configured TCP port (default 5010). The Single Station then publishes its archive segments to the PH using the same archive engine that the OS Server uses, so the historical view on the PH Information Server is identical regardless of the source OS topology.

6. Configuration Parameters and Archive Tuning

6.1 CAS Archive Tag Configuration

The CAS archive tag configuration in WinCC Explorer is governed by the following parameter set. These values should be tuned to the plant's regulatory and operational retention requirements.

6.2 Storage Sizing Heuristic

Use the following approximation to size the CAS storage volume before procurement. For three-phase line current at the secondary of the MV transformer this is single-phase apparent power per phase; for a one-line bus the apparent power is kVA = √3 × V_LL × I_line / 1000. The historian sizing is unrelated to electrical sizing and is governed entirely by tag count, sample rate, compression, and retention:

S_bytes ≈ N_tags × cycle_seconds_per_day × retention_days × B_per_sample × (1 / compression_ratio)

where N_tags is the number of archived tags, cycle_seconds_per_day is (86400 / sample_period_s), and B_per_sample is 8 bytes for a 64-bit floating-point value plus metadata overhead. A typical 10,000-tag plant at 1 s sample, 365 days retention, 1:10 compression yields approximately:

S ≈ 10000 × 86400 × 365 × 16 × 0.1 ≈ 5.04 × 10¹² bytes ≈ 5 TB

Add 20 % headroom for the swap-out buffer and the SQL transaction log. For a redundant CAS pair, double the volume and mirror at the storage level (RAID 5/6 or RAID 10).

7. Verification Procedure

Use the following checks to verify a CAS (or PH) deployment after the topology change is complete.

1. Archive enumeration. On the CAS/PH server open WinCC Explorer and navigate to Computer → Properties → Archive. The number of archive segments listed must equal the number configured on the OS Server (or Single Station in the PH case). Zero segments indicates the Single Station limitation is still in effect or the OS Server is unreachable on the multi-client segment.
2. Tag-level round trip. Force a value change on a controlled tag in the AS and confirm the change is visible in the CAS trend within two sample cycles.
3. Alarm round trip. Trigger a priority-3 alarm from the AS and confirm the message is present in the CAS long-term alarm log with the correct timestamp and acknowledgement state.
4. Redundancy switch-over (if a CAS pair or PH pair is deployed). Disable the primary service and confirm the standby takes over without loss of the in-flight archive segment.
5. Backup and restore. Run a CAS/PH backup job, simulate a segment loss, and restore from the backup. The restore point must match the last archived timestamp before the simulated loss.

8. Troubleshooting Matrix

9. Common Engineering Pitfalls

• Assuming the CAS can attach to a Single Station because they are both "WinCC". The archive interface is fundamentally different and the pairing will silently fail with no segments enumerated.
• Co-locating the CAS on the ES or on an OS Server to save a PC. The CAS service writes to the system volume under load and will conflict with WinCC Runtime and SIMATIC Manager.
• Skipping the domain/credential alignment. CAS runs as a Windows service; if the service user cannot authenticate to the OS Server's archive share, the connector will enumerate segments but never read data.
• Specifying StoragePlus as a "Single-Station-friendly" alternative. StoragePlus V7.1 has the same Single-Station limitation as CAS; only the Process Historian (V8.0+) supports Single Stations directly.
• Under-sizing the SQL transaction log. The default SQL log is 1 MB on a fresh install. For a 10,000-tag, 1 s plant this fills in minutes and the CAS archive write stalls.

10. Field-Proven Commissioning Checklist

1. Confirm the OS topology – Single Station or Server-Client – and the PCS 7 version before ordering the central archive node.
2. For Server-Client, order one Windows Server for the OS Server role, N Windows clients for the OS Client role, and one Windows Server for the CAS (or PH) role. For Multi-Single-Station with a PH, order one Windows Server for the PH role plus the N Single Station PCs.
3. Apply the V8.0 image to all OS and engineering nodes if the migration is chosen.
4. Install the PH (or CAS) on a dedicated PC. Apply all Windows security updates offline before joining the plant network.
5. Configure the PH (or CAS) connector, point it at the OS Server(s) or Single Station(s), and verify the archive segments enumerate.
6. Tune the archive cycle, compression, and retention per the regulatory retention schedule.
7. Run a 24-hour burn-in, verify tag and alarm round trips, then sign off.