Maximum OS Client Limits and Architecture Sizing for Redundant OS Servers in PCS 7 V8.2

In SIMATIC PCS 7 V8.2, a single OS Server or redundant OS Server pair supports a maximum configuration limit of 40 OS Clients connected concurrently. Across a distributed multi-server plant architecture, the total system limit is 64 OS Clients (or up to 128 Web Navigator clients). An architecture comprising 1 Engineering Station (ES), 1 redundant OS Server pair (OS Server and OS Standby), and 10 OS Clients operates safely well within system capacity limits and faces no performance or configuration constraints, requiring only standard OS Client floating runtime licenses.

Official PCS 7 V8.2 Configuration Limits and Sizing Guidelines

The official configuration boundaries for SIMATIC PCS 7 V8.2 are published in the Siemens technical documentation SIMATIC PCS 7 Licenses and Configuration Limits (V8.2), Chapter 4. Sizing guidelines dictate controller, server, client, and tag ceilings to guarantee real-time determinism and sub-second faceplate response times.

These limits establish the tested and warrantied performance envelope for industrial plants operating 24/7.

Client Capacity: Single Redundant Pair vs Multiproject Limits

PCS 7 distinguishes between single-server limits and plant-wide multi-server limits:

Per Redundant OS Server Pair: A maximum of 40 OS Clients can be connected concurrently to one server pair. These clients receive cyclic process data, trend updates, and alarm messages directly from the server pair's tag runtime.
Plant-Wide Multi-Station Limit: In a large facility with multiple OS Server pairs (e.g. Server Pair 1 for Utilities, Server Pair 2 for Synthesis), the plant bus can host up to 64 standard OS Clients (multi-clients accessing all server packages).
Web Navigator Clients: For remote monitoring, a dedicated Web Server supports up to 128 Web Clients without loading the primary OS Servers.

Evaluating the 10-Client Redundant Server Architecture

The user's planned architecture—comprising 1 Engineering Station (ES), 1 Redundant OS Server Pair (Master and Standby), and 10 OS Clients—is an optimal, standard industrial topology:

Capacity Utilization: 10 clients represent only 25% of the 40-client ceiling, leaving substantial headroom for plant expansion.
Fault Tolerance: If OS Server 1 fails or is restarted for maintenance, all 10 OS Clients switch over bumplessly to OS Server 2 (Standby) within 1 to 2 seconds.
Process Object Scope: A single server pair comfortably manages up to 3,000 to 5,000 Process Objects (POs) and up to 120,000 archive tags.

Terminal Bus Network Bandwidth and Hardware Sizing

To support 10 active OS Clients smoothly:

Switched Ethernet: Connect all clients and servers to a dedicated, switched 100 Mbit/s or 1 Gbit/s Terminal Bus.
Network Segregation: The Terminal Bus must remain completely segregated from the Plant Bus (AS-OS network) to prevent heavy SQL client queries from impacting deterministic AS communications.
Licensing: Each client station requires an OS Software Client V8.2 floating license (6ES7658-2CX28-0YB5).

Comparison of PCS 7 System Configuration Limits Across Releases

The table below contrasts configuration limits across PCS 7 versions:

Architectural ParameterPCS 7 V8.1PCS 7 V8.2PCS 7 V9.0 / V9.1Max OS Clients per Server Pair40 Clients40 Clients40 ClientsMax OS Clients per Multiproject64 Clients64 Clients64 ClientsMax Web Clients (WebNavigator)100 Clients128 Clients150 ClientsMax Process Objects per Server3,000 PO (Standard) / 5,000 PO5,000 PO (Standard) / 8,500 POUp to 12,000 PO (CPU 410)Max Archive Variables per Server120,000 Variables120,000 Variables120,000 Variables

Verification

Verify system sizing and client connectivity in runtime:

Power up the redundant server pair and all 10 OS Clients.
In WinCC Explorer on Server 1, open Tag Management; verify connected client stations in internal tags (@ClientCount).
Open multiple faceplates simultaneously across all 10 clients; verify faceplate opening latency remains under 1 second.
Deactivate Server 1 runtime; verify all 10 clients switch to Server 2 without alarm loss or operator disruption.
Reactivate Server 1; verify redundant synchronization completes and normal redundancy status is restored.

Limitations and Operational Precautions

Review the following network precautions:

No Terminal Bus in NetPro: Never configure the Terminal Bus in STEP 7 NetPro; it is managed strictly by Windows networking.
Domain Architecture: For 10 clients, deploying a centralized Windows Active Directory Domain Controller (AD DC) is strongly recommended to manage user accounts and security policies uniformly.
Commercial Procurement Guardrail: Validate that 10 OS Client license packages (6ES7658-2CX28-0YB5) and appropriate SFC Visualization licenses (6ES7652-0XD28-2YB5) are included in the procurement schedule.

Engineer FAQ

What is the maximum number of OS Clients a single redundant server pair can handle?

A single OS Server or redundant OS Server pair supports up to 40 standard OS Clients connected concurrently.

Why do some PCS 7 architecture diagrams show multiple server pairs for fewer clients?

Multiple server pairs are typically introduced to segregate distinct plant areas (e.g. Unit 1 vs Unit 2) or divide large Process Object counts (exceeding 5,000 PO), rather than due to client count limitations.

Do OS Clients require AS-OS connection licenses?

No. OS Clients communicate only with the OS Servers over the Terminal Bus. Only the OS Servers require AS communication licenses (such as S7-REDCONNECT).