Can WinCC Clients Communicate Directly with PLCs When the OS Server Fails

WinCC OS Clients cannot communicate directly with Automation Stations (AS/PLCs) when an OS Server fails because clients lack direct Plant Bus communication channels, S7 connection drivers, and local process databases. In PCS 7 client-server architecture, clients rely entirely on the Terminal Bus to query tags, alarms, and archives via server data packages. To operate an OS Client directly against the AS during an extended server outage, the client station must be re-engineered as an independent Single-Station OS with physical Plant Bus access, local S7 connections, Single Station runtime licenses, and a full OS compilation.

PCS 7 Distributed Architecture: Plant Bus vs. Terminal Bus

In SIMATIC PCS 7 multi-user distributed architectures, network separation and functional role division are foundational design principles:
1. Plant Bus (Industrial Ethernet): Interconnects Automation Stations (S7-400 / S7-410 controllers) and OS Servers via CP 1623 communication processors or Softnet Industrial Ethernet adapters. High-frequency cyclic S7 communication and time synchronization frames operate across this subnet.
2. Terminal Bus (Local Area Network): Interconnects OS Servers and OS Clients. OS Clients run thin WinCC Client applications that display graphics and dispatch operator commands to the server using Microsoft RPC, DCOM, and TCP/IP.
3. Server Data Packages: OS Clients possess no local tag management database, no Tag Logging engine, and no direct communication drivers (e.g., Named Connections). Instead, they load a compiled Server Data Package (*.pck) that routes all tag requests to the OS Server runtime processes (CCDataServer.exe, CCAlgRtServer.exe).

When a non-redundant OS Server fails, all OS Clients referencing its server package lose communication instantly, displaying disconnected yellow/gray process values and inactive faceplates.

Why OS Clients Cannot Autonomously Poll PLCs

An OS Client cannot simply "switch its target IP" to an AS controller due to architectural constraints:
* No S7 Channel Driver: OS Client projects run a lightweight runtime environment that lacks configured SIMATIC S7 Protocol Suite drivers or Named Connection endpoints.
* Physical Network Isolation: In standard industrial control systems, OS Clients are physically connected only to the Terminal Bus switch and lack physical or routed IP access to the Plant Bus network.
* Licensing Model: OS Clients are licensed with WinCC Client (RT) licenses, which explicitly forbid opening local S7 communication channels. Direct controller communication requires a full WinCC Single Station (RT) or WinCC Server (RT) license.

Emergency Procedure to Convert an OS Client into a Single-Station OS

If a plant experiences a catastrophic server hardware failure without a redundant partner, an OS Client can be converted into an emergency standalone Single-Station OS via the following engineering steps on the Engineering Station (ES):

Establish Physical and Logical Plant Bus Connectivity:
* Install an Industrial Ethernet network adapter in the client PC or patch the client network interface into the Plant Bus switch.
* Assign a valid static IP address on the Plant Bus subnet.
Reconfigure Hardware Configuration in SIMATIC Manager:
* Open the client PC Station in HW Config.
* Remove the WinCC Client Application.
* Insert a standard WinCC Application into Slot 1.
* Insert an IE General or CP 1623 adapter into Slot 2 and assign it to the Plant Bus subnet.
* In NetPro, configure Named S7 Connections between the new WinCC Application and the AS controllers.
* Save and compile NetPro and HW Config.
Compile and Download OS:
* Open SIMATIC Manager and initiate an OS compilation for the newly configured OS Single Station to map tags, alarms, and SFCs directly from the AS.
* In OS Properties, set the target path to the client PC.
* Execute an OS download to the client machine.
License Migration:
* Open Automation License Manager (ALM) on the ES and transfer a temporary or floating WinCC Single Station RT (or RC) license and PCS 7 OS RT PO licenses to the client machine.
Activate Runtime:
* Launch WinCC Explorer on the converted PC and activate Runtime.

Verification

In Windows Network Connections on the converted station, verify link connectivity and ping response to the AS CPU IP addresses.
Open WinCC Channel Diagnosis in WinCC Runtime. Confirm that the SIMATIC S7 Protocol Suite -> Named Connections channel reports a green check mark with State: OK.
Confirm that process graphics display live dynamic values, alarms appear in the alarm banner, and faceplate operator commands successfully manipulate field actuators.

Limitations and Operational Precautions

Converting a client to a single station is an emergency recovery procedure. It requires an available Engineering Station with the complete project source code.
Historical Tag Logging and Alarm Logging archives previously stored on the failed server will remain inaccessible until the server disk or database is recovered.
Always specify redundant OS Servers (Server Pairs) in the initial plant design to guarantee uninterrupted visualization during server hardware failures.

Engineer FAQ

Can a client connect to a redundant standby server automatically?
Yes. If the system was designed with a redundant OS Server pair, clients switch over to the redundant partner automatically within 1 to 3 seconds of a master server failure.
Can I run S7 connections over the Terminal Bus using a router?
While network routing is technically feasible at the IP level, Siemens PCS 7 guidelines strictly prohibit routing Named S7 Connections across the Terminal Bus due to real-time deterministic communication requirements.
What happens to alarms when running an emergency single station?
The emergency single station will receive new real-time alarms generated by the AS from the moment it comes online, but historical alarm journals from prior days remain on the offline server disk.

Technical References

SIMATIC PCS 7 OS Process Control System; Client-Server Architecture Manual (A5E41356262-AA).
Siemens Industry Online Support Entry ID 109801081: PCS 7 Redundancy and Multi-Station Architectures.
SIMATIC WinCC V7/V8 Communication Manual: SIMATIC S7 Protocol Suite.