Resolving Dual 'Active Master' Conflict in WinCC OS Single Station Time Synchronization

In SIMATIC PCS 7, when two WinCC OS Single Stations connected to the Plant Bus both display the internal status @TimeSyncDevice1State = 'Active Master' instead of one Active Master and one Standby Master, a network partition or protocol misconfiguration exists where neither station receives the Layer 2 ISO time broadcast frames of the other. To resolve this conflict, verify that the CP 1623 hardware configuration in STEP 7 HW Config has SIMATIC time synchronization enabled and is downloaded to both PC stations, ensure intermediate network switches pass Layer 2 multicast frames, and verify that the WinCC Time Synchronization editor assigns matching Access Points.

How WinCC Time Synchronization Master Negotiation Operates

In SIMATIC PCS 7 architectures featuring multiple Operator Stations (such as dual OS Single Stations communicating with redundant S7-400H / CPU 410-5H automation systems), system time must remain uniform across every node.

When external GPS master clocks (like SICLOCK) are omitted, WinCC provides an integrated master-standby time synchronization mechanism configured via the Time Synchronization editor in WinCC Explorer:
* Both OS stations are configured as potential Time Masters over their Plant Bus communications processors (CP1623(ISO)).
* Under proper operation, the station that starts runtime first claims mastership, setting internal tag @TimeSyncDevice1State to Active Master.
* It broadcasts periodic Layer 2 ISO time telegrams across the Industrial Ethernet Plant Bus.
* The secondary station listens on the bus, detects the active master's broadcasts, and automatically transitions its state to Standby Master, continuously synchronizing its local clock from the active partner.

If both stations report Active Master, the automatic negotiation has broken down, creating two unsynchronized time masters.

Root Causes of Dual 'Active Master' Split-Brain Conditions

A dual Active Master condition indicates that each station believes it is the sole master because it never receives time telegrams from its peer:

1. Incomplete HW Config Download to CP 1623

While Time Synchronization is enabled in the WinCC Explorer graphic interface, the physical CP 1623 communications coprocessor must also be instructed by the operating system to process SIMATIC time frames. If the STEP 7 Hardware Configuration (HW Config) for the PC Station was not compiled and downloaded directly into the Station Configuration Editor (SCE), the CP 1623 firmware drops incoming Layer 2 time broadcast frames.

2. Network Switch Multicast Filtering

SIMATIC time synchronization over Industrial Ethernet does not use standard IP packets; it transmits raw Layer 2 ISO multicast/broadcast frames. If intermediate managed Industrial Ethernet switches (such as SCALANCE X-200 or X-300) have aggressive broadcast storm limiters enabled, or if the two OS stations reside in separate VLANs without Layer 2 bridging, the time telegrams are dropped by the switch fabric.

3. Access Point Parameter Mismatch

In WinCC Time Synchronization, the access point must match the physical protocol binding. If one station uses CP1623(ISO) while the other uses a different access point or mismatched cycle times, peer discovery fails.

Comparison Table - Time Synchronization State Tags and Roles

The following table details the key WinCC internal system tags that govern time synchronization status.

System Tag NameExpected Normal StateDual Master Fault StateDiagnostic Meaning@TimeSyncDevice1StateActive Master (OS 1) / Standby Master (OS 2)Active Master on both stationsStations cannot detect peer time telegrams@TimeSyncDevice2StateSlave / InactiveVariesStatus of secondary network access point (e.g., Terminal Bus)@TimeSyncMasterNameName of active OS stationLocal computer name on bothEach station considers itself the authoritative source@TimeSyncStatusOK (0)Warning / FaultGeneral status of the WinCC Time Synchronization daemon

Step-by-Step Diagnostic and Resolution Procedure

Follow this procedure to resolve the dual Active Master conflict and establish coordinated master/standby synchronization:

Verify CP 1623 Properties in STEP 7 HW Config:
* Open the PC Station for both OS stations in SIMATIC Manager HW Config.
* Double-click the CP 1623 in the virtual rack.
* Navigate to the Time-of-Day Synchronization tab.
* Verify that Activate time-of-day synchronization is checked.
* Ensure the mode is set to SIMATIC Mode (or NTP Mode if an IP time server is used).
* Save and compile HW Config.
Download PC Station Configuration via SCE:
* Set the PG/PC Interface access point S7ONLINE to PC internal.local.1.
* In HW Config, select PLC -> Download to Current Module for each OS station.
* In the Station Configuration Editor on both PCs, confirm that the CP 1623 accepts the configuration without errors.
Verify WinCC Time Synchronization Settings:
* Open WinCC Explorer on both OS stations.
* Open the Time Synchronization editor.
* Under Access Point 1:
- Check Time synchronization via Access Point 1.
- Select CP1623(ISO) from the dropdown.
- Check Master on both stations.
- Set the broadcast interval to 10 seconds.
- Click OK to save.
Verify Physical Network and Port Mirroring:
* If both stations connect through SCALANCE switches, verify that both switch ports reside within the identical VLAN.
* Disable broadcast storm rate limiters for Layer 2 multicast frames on the switch ports connecting the CP 1623 cards.
* To verify packet transmission, configure a port mirror on the switch and use Wireshark on a diagnostic laptop to capture Industrial Ethernet frames with EtherType 0xFD or 0x8892.

Verification

Confirm that the time synchronization hierarchy is established:

Inspect @TimeSyncDevice1State Values:
* In WinCC Runtime, open an internal tag monitor or Graphics Designer screen bound to @TimeSyncDevice1State.
* Confirm that one OS station displays Active Master and the second OS station displays Standby Master.
Test Failover Resiliency:
* Deactivate WinCC Runtime on the Active Master station.
* Verify that the secondary station immediately transitions from Standby Master to Active Master within 15 seconds.
* Reactivate the first station. Confirm that it enters runtime as Standby Master without disturbing the active clock.
Verify AS Controller Synchronization:
* Go online with the S7-400H CPU in SIMATIC Manager.
* In Module Information -> Time System, confirm that the CPU indicates Synchronized from the CP 1623.

Limitations and Operational Precautions

Prohibition on Redundant OS Server Mismatches: In redundant OS Server pairs, time synchronization must be configured strictly identical on both partner servers; differing broadcast intervals cause cyclic master toggling.
Terminal Bus Isolation: Never enable SIMATIC mode on the Terminal Bus. SIMATIC Layer 2 ISO broadcasts are intended strictly for the Plant Bus. The Terminal Bus must use NTP or Windows W32Time.
Firmware Consistency: Ensure CP 1623 cards in both stations are running the identical firmware version.

Engineer FAQ

Can two OS Single Stations communicate time sync if they have different project names?

Yes. Time synchronization master-standby negotiation operates at the hardware protocol level via ISO broadcast frames, independent of WinCC project names or database structures.

What happens to the automation controller if both stations broadcast as Master?

If both stations broadcast time telegrams with slightly different clock values, the S7-400 CPU will experience frequent, erratic clock jumps back and forth by several milliseconds, creating jitter in alarm timestamps.

Why not use an external SICLOCK or GPS receiver instead?

Deploying a dedicated external GPS master (like SICLOCK TC 100/400) is the industry best practice. It eliminates master negotiation between PCs entirely, allowing all OS stations and PLCs to operate as synchronized slaves.

Can Wireshark capture SIMATIC time telegrams?

Yes. Capturing on the physical network card shows Industrial Ethernet frames with SAP (Service Access Point) 0x04 or 0x06 representing SIMATIC time synchronization broadcasts.