Restoring WinCC Redundant S7-400H Communication After SCALANCE Switch Replacements

When SIMATIC WinCC OS loses communication with a redundant S7-400H controller following the replacement of plant bus network switches, the failure typically stems from unconfigured managed switch parameters rather than PLC software defects. Replacement SCALANCE switches default to factory settings, which lack required Spanning Tree (RSTP/MRP) ring configurations, fail to unblock redundant S7-RedConnect communication paths, or introduce port forwarding delays. To resolve this, restore SCALANCE ring manager settings, verify Station Configuration Editor status, and check WinCC Channel Diagnosis under Named Connections.

Fault-Tolerant AS-OS Communication and S7-RedConnect Architecture

In high-availability SIMATIC PCS 7 installations, the process control system relies on fault-tolerant Automation Stations (S7-400H or CPU 410-5H) paired with redundant WinCC OS Servers. Communication between AS and OS operates across redundant Industrial Ethernet plant buses using Named Connections managed by SIMATIC NET S7-RedConnect.

The physical and logical topology comprises:
* Two Subnets / Redundant Ring: Dual Industrial Ethernet cables connecting AS CP 443-1 communication processors to managed network switches (e.g. SCALANCE X-200, X-300, or XM-400).
* OS Communications Processors: CP 1623 (6GK1162-3AA00) or CP 1613 A2 (6GK1162-3AA00 / 6GK1161-3AA01) installed in each OS Server, or standard Industrial Ethernet adapters utilizing the S7-RedConnect software package.
* S7-RedConnect Logical Layer: Maintains four active physical sub-paths simultaneously (OS Path A -> CPU 0, OS Path A -> CPU 1, OS Path B -> CPU 0, OS Path B -> CPU 1). If any cable, switch, or controller rack fails, S7-RedConnect switches communication seamlessly in under 100 milliseconds without interrupting WinCC tag updates or triggering operator alarms.

Why Replacing Network Switches Interrupts S7-400H Connections

When industrial plant switches fail (e.g. due to auxiliary 24 V DC power supply faults or lightning surges) and maintenance personnel install replacement switches fresh from factory packaging:
1. Unconfigured Factory Defaults: Brand-new SCALANCE switches ship with default unmanaged or flat Layer-2 configurations. They do not have High-Speed Redundancy Ring (HRP) or Media Redundancy Protocol (MRP) enabled.
2. Network Broadcast Storms and Ring Loops: In a ring topology, connecting unmanaged switches forms an active network loop. Without an active Redundancy Manager (RM), broadcast frames circulate endlessly, flooding the network and causing 100% bandwidth collapse across CP 443-1 interfaces.
3. Port Spanning Tree Delays (RSTP): If Rapid Spanning Tree Protocol (RSTP) is enabled without 'Edge Port' parameterization, switch ports delay link forwarding for up to 30 seconds upon detecting link transitions, causing S7-RedConnect handshake timeouts.
4. VLAN Segmentation Loss: If plant bus and terminal bus shared the physical switch hardware using IEEE 802.1Q VLAN IDs, replacing the switch places all traffic into the default VLAN 1, creating subnet mismatch.

Diagnostic Evaluation via WinCC Channel Diagnosis and Simatic Shell

To pinpoint why WinCC cannot reconnect:
* WinCC Channel Diagnosis (WinCC Channel Diagnosis.exe):
- Open the diagnostic tool on the WinCC OS Server.
- Expand SIMATIC S7 Protocol Suite -> Named Connections.
- Inspect the connection state:
- State: Disconnected (Red cross) indicates zero physical or logical paths can reach the controller.
- Error Code 16#4104 or 16#4110 indicates physical MAC/IP unreachable or NetPro connection ID mismatch.
* Simatic Shell Diagnostics:
- Open Windows Explorer -> This PC -> Simatic Shell.
- Right-click and open Settings. Verify that the network adapter assigned to Simatic Shell corresponds to the active Terminal Bus, not the Plant Bus.
* Station Configuration Editor:
- Check the OS Server desktop taskbar: Ensure the Station Configuration Editor indicates RUN mode with all CP 1623 and WinCC Application components healthy.

Industrial Ethernet Switch Configuration Comparison Matrix

The following matrix compares factory default settings against required PCS 7 plant bus switch settings:

Parameter / FeatureFactory Default (Out of Box)Required for S7-400H Plant BusFailure Consequence if UnsetRing RedundancyDisabled (Flat Switch)HRP / MRP Redundancy Manager or ClientCatastrophic broadcast loopRing PortsStandard PortsDesignated Ring Ports (e.g. Port 1 & Port 2)Ring break detection failsAging Time (MAC Table)300 seconds30 seconds (or fast flush)Stale MAC routes during CPU failoverTransmission RateAuto-negotiation (Auto / Auto)100 Mbps Full Duplex (Fixed on AS ports)Duplex mismatch causing packet lossSpanning Tree Edge PortDisabledEnabled on end-device ports (AS / OS)30-second connection timeoutFault Signaling ContactDisabledConfigured to monitor power and ring statusUndetected redundant link breaks

Step-by-Step Remediation Procedure for Replaced SCALANCE Switches

Follow this procedure to reconfigure replacement switches and restore AS-OS communication:
1. Isolate the Replacement Switches:
- Disconnect the redundant ring cable temporarily to prevent network broadcast loops until ring parameters are configured.
2. Access Web-Based Management (WBM):
- Connect an engineering laptop to a spare port on the new SCALANCE switch.
- Use the Primary Setup Tool (PST) or Siemens PRONETA to discover the switch and assign its configured plant bus IP address and subnet mask.
- Open a web browser, navigate to the switch IP address, and log in with administrative credentials.
3. Configure Ring Redundancy:
- In WBM, navigate to Layer 2 -> Ring Redundancy.
- If the switch is designated as the master: Set Redundancy Mode to HRP Manager or MRP Auto/Manager.
- If the switch is a secondary node: Set Redundancy Mode to HRP Client or MRP Client.
- Assign the exact physical ports connected to the ring cables as Ring Port 1 and Ring Port 2.
4. Configure End-Device Ports:
- Under Layer 2 -> Ports: Select the ports connected to the S7-400H CP 443-1 modules and OS Server CP 1623 cards.
- Enable Edge Port (or PortFast).
- Set transmission speed to 100 Mbps Full Duplex if CP 443-1 is fixed, or leave Auto-negotiation if Gigabit fiber is deployed.
5. Reconnect Ring Cables and Re-initialize Communication:
- Reconnect the redundant ring cable. Verify the switch RM or Fault LED clears to green.
- On the WinCC OS Server, open Windows Command Prompt and restart the communications driver or reboot the OS Server if Named Connections do not automatically re-sync.

Verification

Verify complete restoration of fault-tolerant communication:
1. Open WinCC Channel Diagnosis: Under Named Connections, verify the connection displays a green checkmark and status Ready / OK.
2. Open WinCC Runtime: Confirm that all process graphics populate with live, dynamic values and that disconnected tags (???) completely vanish.
3. Test Fault Tolerance (H-Failover):
- In STEP 7, trigger a CPU redundancy test (switch active master from CPU 0 to CPU 1).
- Confirm that WinCC process values continue updating without interruption or alarm banner disconnection.

Limitations and Operational Precautions

Never operate an Industrial Ethernet ring without exactly ONE Redundancy Manager (RM); having zero managers causes loops, while having two managers causes network partitioning.
When replacing switches on a live plant, disconnect redundant ring links before connecting power to prevent transient loops from taking down operating controllers.
Ensure replacement switches match the environmental specifications and firmware release approved for the plant bus architecture.

Engineer FAQ

Why did the MP 377 touch panel also lose communication when the switch was replaced?

Touch panels (e.g. SIMATIC MP 377 6AV6644-0AA01-2AX0) connect directly to the plant bus switch. When the switch defaulted to unmanaged mode without ring control, broadcast storms saturated the panel's 10/100 Mbps Ethernet port, causing TCP timeout.

What does Error 16#4104 mean in WinCC Channel Diagnosis?

Error 16#4104 indicates 'Connection to partner station could not be established'. It signifies that the physical Ethernet frame cannot reach the remote MAC or IP address due to cabling, switch port blocking, or IP subnet mismatch.

Can I configure SCALANCE switches directly from STEP 7 / PCS 7?

Yes. If the SCALANCE switch is integrated into STEP 7 HW Config, its IP address, device name, and ring redundancy settings can be downloaded directly over Industrial Ethernet using the 'Download to Device' function.