How to Troubleshoot Intermittent WinCC Redundancy Failures and Diagnostic Logs in SIMATIC PCS 7

Intermittent, recurring redundancy alarms in SIMATIC PCS 7 OS server pairs typically stem from micro-disconnections on the terminal bus, network adapter energy-saving throttling, or binding misconfigurations in SIMATIC Shell. To identify the root cause, inspect the diagnostic files 'SIMATIC_S7_Protocol_Suite_*.log' and 'WinCC_Sys_01.log' located in the WinCC Diagnostics directory. Verify that the terminal bus network adapter is listed first in Windows network bindings, confirm internal system tags (@RM_MASTER) reflect proper master-standby states (1 and 0), disable 'Allow the computer to turn off this device to save power' in network card properties, and verify physical cable seating.

Root Causes of Intermittent Redundancy Alarms in OS Server Pairs

In a SIMATIC PCS 7 distributed architecture, Operator Station (OS) redundant server pairs maintain continuous mutual health monitoring across two independent channels: the Terminal Bus (TCP/IP network connection) and an optional Redundancy Link (dedicated network cable or serial connection).

When a plant experiences sporadic, recurring redundancy alarms—such as Redundancy Control: Connection to partner server interrupted followed minutes later by Partner server returned:
* The issue is rarely a permanent hardware failure, but rather transient packet loss or latency spikes.
* Loose Physical Cabling: A partially seated RJ45 jack, damaged patch cable, or vibrating switch port causes microsecond link drops that trigger WinCC Redundancy Watchdogs.
* Network Interface Card (NIC) Power Throttling: Modern Windows Server operating systems enable 'Green Ethernet' and power-saving sleep modes on physical NICs by default, placing inactive adapters into low-power states.
* SIMATIC Shell Adapter Binding: If the terminal bus adapter is not prioritized at the top of the Windows network binding order, internal broadcast heartbeats are routed intermittently to secondary virtual adapters.

Analyzing WinCC Diagnostics Logs: Protocol Suite and System Logs

To isolate the exact moment and mechanism of redundancy disconnection, analyze the diagnostic text files stored on both OS servers in:
C:\Program Files (x86)\Siemens\WinCC\Diagnostics\

Key log files to inspect:
1. SIMATIC_S7_Protocol_Suite_01.log:
* Records low-level S7 communication events between the OS and the AS controllers.
* Look for entries reporting: Connection error: 0x80004005 or TCP/IP disconnected. If errors coincide with redundancy alarms, the issue affects physical plant bus communications.
2. WinCC_Sys_01.log:
* Logs WinCC core system events, process crashes, and database connection timeouts.
3. Redundancy.log:
* Explicitly details state transitions between Master and Standby.
* Logs specific trigger reasons, such as: Partner monitoring: Ping failed, Server state change: STANDBY -> MASTER, or Network failure detected on interface [IP_Address].

Network Adapter Power Management and Hardware Binding Priority

To prevent Windows from suspending network adapters during operation:

Disable NIC Power Management:
* Open Windows Device Manager (devmgmt.msc).
* Expand Network adapters, right-click the Terminal Bus physical NIC, and select Properties.
* Open the Power Management tab.
* Uncheck: Allow the computer to turn off this device to save power.
* Open the Advanced tab: disable Energy Efficient Ethernet, Green Ethernet, and IPv4 Checksum Offload (if using virtualization).
Configure Windows Network Binding Order:
* In Windows Network Connections, press Alt and select Advanced > Advanced Settings....
* Under Adapters and Bindings, ensure the Terminal Bus adapter appears at the absolute top of the list, followed by the Plant Bus adapter.

Validating SIMATIC Shell Settings and @RM_MASTER Status Tags

To ensure proper heartbeat routing and status coordination:

Inspect SIMATIC Shell:
Open Windows Explorer and right-click SIMATIC Shell > Settings....
Confirm that the selected network adapter is the physical Terminal Bus card.
Verify that all participating servers, clients, and the ES appear with status Available or Good.
Monitor Redundancy System Tags in WinCC Tag Management:
@RM_MASTER: Indicates the master server. Exactly one server must report @RM_MASTER = 1, while the partner reports @RM_MASTER = 0. Both servers reporting 1 simultaneously indicates a severe split-brain condition.
@RedundancyState: Reports overall operational health (1 = Fully Synchronized, 2 = Synchronizing, 0 = Fault/Disconnected).
@RedundantServerState: Indicates station role (1 = Master, 2 = Standby).

Comparison of WinCC Redundancy Diagnostic Log Indicators

The table below outlines common error messages found in WinCC diagnostic logs and their corresponding physical causes.

Diagnostic Log FileTypical Error StringRoot CauseCorrective ActionRedundancy.logPing to partner failed: TimeoutPhysical terminal bus disconnection or switch latencyReseat network cables; replace damaged patch leadSIMATIC_S7_*.logConnection error to device [AS_Name]Plant bus CP failure or cable disconnectionInspect CP 1623 card and industrial Ethernet switchesWinCC_Sys_01.logDatabase synchronization timeoutSQL Server mirroring port blocked or heavy disk I/OCheck SQL Server health and network bandwidthWindows Event LogThe network link is down (Event ID 27)Physical cable unseated or NIC power sleep modeDisable NIC power management; check RJ45 latch

Verification

To confirm that WinCC redundancy is completely stable:
1. Open the WinCC alarm journal on both OS servers: verify that no recurring Redundancy Control disconnection alarms appear over a 72-hour operational window.
2. In WinCC Tag Management, verify that internal tag @RedundancyState remains constant at value 1 (Synchronized).
3. Confirm that @RM_MASTER reports 1 on Server 1 and 0 on Server 2.
4. Perform a controlled network test: disconnect the terminal bus cable momentarily. Confirm that an alarm is generated, and upon reconnection, archive synchronization completes automatically and clears all alarms.

Limitations and Operational Precautions

Never activate WinCC Runtime on an OS Server directly from the Engineering Station (ES); testing runtime on the ES creates conflicting server packages and disrupts live server redundancy.
Always use industrial-grade, shielded twisted-pair (STP) Cat6 cables with latching RJ45 connectors on all OS servers to prevent vibration-induced disconnects.
When replacing network cards or servers, ensure both partner stations use identical network adapter hardware and firmware revisions.
Do not alter internal redundancy tags (@RM_MASTER, @RedundancyState) manually in Tag Management; forcing values manually corrupts the redundancy state machine.

Engineer FAQ

Why did pushing the network cable firmly solve the recurring alarms?

In high-speed Gigabit Ethernet networks, a partially latched RJ45 connector can maintain electrical continuity for basic TCP packets while experiencing microsecond dropouts during heavy traffic bursts. WinCC Redundancy monitors heartbeat telegrams continuously; even a transient 500 ms link drop prompts the system to declare partner communication failure.

What does it mean if @RM_MASTER is 1 on both servers?

If both servers report @RM_MASTER = 1, a 'split-brain' condition has occurred. Both servers believe the other is dead, causing both to act as independent masters. This leads to unsynchronized alarm states and database inconsistencies. Reconnecting the terminal bus and restarting the standby server resolves the condition.