Understanding Communication Paths for WinCC Reference OS Stations: Direct AS Link vs Base OS Data

In SIMATIC PCS 7, a WinCC Reference OS station (WinCC Application Ref) establishes its own direct, autonomous S7 communication link to the automation station (AS) over the Plant Bus using its local communications processor (such as CP 1623 or IE General), rather than retrieving process telemetry from the Base OS. While the Reference OS inherits its process graphics, compiled tags, and plant hierarchy from the Base OS during engineering and download, in runtime it operates as an independent Single Station. If the Base OS crashes, reboots, or suffers hardware failure, the Reference OS continues running with 100% active process visualization and controller communication.

Runtime Communication Architecture of WinCC Reference OS Stations

Process automation engineers frequently confuse a Reference OS with an OS Client:
* The OS Client Paradigm:
* An OS Client possesses no direct connection to the S7-400 controller.
* It communicates exclusively with an OS Server over the Terminal Bus.
* If the OS Server crashes, the OS Client immediately loses process visualization (displays # bad quality symbols).
* The Reference OS Paradigm:
* A Reference OS is an OS Single Station.
* It inherits its project configuration from a Base OS during engineering.
* At runtime, it communicates directly with the S7-400 controller over the Plant Bus.
* It has zero runtime dependency on the Base OS!

Plant Bus Integration: CP 1623 Direct Named Connections

To establish direct controller communication for the Reference OS:

Hardware Configuration in HW Config:
The Reference PC station (SIMATIC PC Station) contains:
1. WinCC Appl. (ref) in Slot 1.
2. CP 1623 (or IE General) in Slot 2, assigned a unique static IP address on the Plant Bus.
Named Connection Assignment:
During Base OS compilation, NetPro establishes Named Connections between the WinCC Application and the S7-400 CPU.
When the Reference OS is downloaded, SIMATIC Manager configures a parallel Named Connection for the Reference OS's CP 1623.
In runtime, the Reference OS polls the S7-400 CPU directly over ISO Transport or TCP/IP.

Engineering Relationship vs Runtime Autonomy (Base OS vs Ref OS)

Differentiating engineering synchronization from runtime execution:

Engineering Phase (Coupled):
The Base OS is the single point of configuration.
Tag compilation, faceplate design, and hierarchy structuring are performed exclusively on the Base OS.
The Reference OS cannot be compiled independently; it updates whenever the Base OS is compiled.
Runtime Phase (Decoupled):
The Reference OS runs its own local Microsoft SQL Server instance (MSSQL$WINCC).
Process alarms, tag logs, and operator inputs are processed locally on the Reference OS.
If the Base OS is powered down for maintenance, the Reference OS experiences zero interruption.

Configuring User Rights, Windows Sharing, and Security Settings

To ensure flawless project download from the ES to the Reference OS:

Synchronize Local Windows Accounts:
* Create an identical Windows user account (Administrator privileges) on ES, Base OS, and Reference OS.
* Ensure passwords match identically across all machines.
SIMATIC Group Memberships:
* Add the project user to local groups: SIMATIC HMI, SIMATIC NET, and SQLServer2008SQLServerUser$<Comp>$WINCC.
User Account Control (UAC):
* In Windows Control Panel, set UAC to Never Notify.
SIMATIC Security Controller:
* Run SIMATIC Security Controller > Repeat Settings on the Reference OS and reboot.

Comparison of Multi-Client vs Reference OS Operational Topologies

The table below compares OS Client and Reference OS operational topologies.

Operational MetricStandard OS ClientReference OS Single StationConnection to AS ControllerNone (Indirect via OS Server)Direct S7 Named Connection via CPSurvival During Server FailureFails (Loses all data)100% Autonomous (Continuous Control)Terminal Bus TrafficHeavy (Continuous tag queries)Minimal (Engineering downloads only)Plant Bus TrafficNoneDirect cyclic polling of S7-400Local SQL Server DatabaseNo (Client cache only)Yes (Full local runtime database)Software Licensing RequiredOS Client LicenseFull OS Single Station Runtime License

Verification

To confirm direct AS communication on a Reference OS:
1. Activate WinCC Runtime on the Reference OS.
2. Open WinCC Channel Diagnosis:
* Expand SIMATIC S7 Protocol Suite > Named Connections.
* Confirm that the connection to the S7-400 controller reports State: OK with telegram counters incrementing.
3. Shut down the Base OS computer completely.
4. Observe the Reference OS screens: confirm that all motor faceplates, valve controls, and analog trends continue updating dynamically without any # symbols or communication fault popups.

Limitations and Operational Precautions

Each Reference OS single station consumes a dedicated S7 connection resource on the S7-400 CPU and CP 443-1; verify controller connection budgets before deploying multiple single stations.
Process alarms acknowledged on the Reference OS do not automatically acknowledge on the Base OS because each single station maintains an independent SQL database.
Always execute OS downloads to the Base OS first, followed by the Reference OS, to maintain version consistency.
Always validate final software licensing, single-station runtime packages, and hardware ordering numbers against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

Does the Reference OS lose data if the Base OS crashes?

No. The Reference OS communicates directly with the controller over the Plant Bus; it has zero runtime reliance on the Base OS.

Can a Reference OS use a standard commercial NIC instead of CP 1623?

Yes. A standard commercial network interface card running SOFTNET-S7 (IE General) is fully supported for Reference OS single stations.

Why did the Reference OS download fail with an access error?

Windows credentials or network share permissions are mismatched. Ensure identical Administrator accounts exist on ES and Reference OS, and run the SIMATIC Security Controller.