Resolving Redundant OS Server S7 Connection Failures and CP 1623 Disconnects in PCS 7

In SIMATIC PCS 7 redundant OS Server architectures, when one server is online while its redundant partner shows all PLC connections disconnected despite successful ICMP ping responses, the failure is caused by S7 connection naming discrepancies in NetPro, unassigned access points in Set PG/PC Interface, or misconfigured Station Configuration Editor slots. Pinging verifies only basic Layer 3 IP connectivity, not Layer 7 S7 application associations. To resolve this, ensure NetPro connection names match exactly, verify S7ONLINE points to CP1623(ISO) in Configured Mode, and inspect SIMATIC_S7_PROTOCOL_SUITE_01.LOG.

Network Diagnostic Fallacy: Layer 3 ICMP Ping vs. Layer 7 S7 Protocol

In high-availability SIMATIC PCS 7 plants utilizing redundant OS Servers (Server A and Server B) equipped with communication processors such as the CP 1623 (MLFB 6GK1162-3AA00), servers interface with multiple S7-400 / S7-410 controllers over a fault-tolerant Industrial Ethernet ring.

A common and perplexing troubleshooting scenario occurs when Server A operates normally, displaying all live process values, while Server B starts WinCC Runtime and indicates that all PLC connections are disconnected (grayed-out I/O fields and broken wire indicators). When the engineer tests network connectivity by pinging the controllers from Server B:
* The command prompt reports: Reply from 192.168.2.10: bytes=32 time<1ms TTL=64.
* The engineer mistakenly concludes that network communications are healthy.

This conclusion overlooks network layering:
1. ICMP Ping (Layer 3 IP): Only proves that the physical Ethernet cable, network switch port, and IP address stack are responsive.
2. S7 Protocol Suite (Layer 7 Application): Requires deterministic S7 communication endpoints: matching ISO-on-TCP Transport Service Access Points (TSAP), authenticated Named Connections compiled in NetPro, valid local SIMATIC NET access points (S7ONLINE), and synchronized Station Configuration Editor indices.

If Server B's CP 1623 is operating in the wrong access mode or NetPro connections were compiled with mismatched symbolic names, the S7-400 CPUs reject the connection request immediately, leaving WinCC disconnected despite 100% ping success.

Redundant OS Server Communication Parameter Matrix

Verify system configuration parameters across redundant OS Server pairs:

Diagnostic Layer / SettingHealthy Server A (Master)Failing Server B (Standby)Required Symmetric SettingNetPro S7 Connection NameOS_SRV_A#AS_Unit1OS_SRV_B#AS_Unit1Must match target AS connection name compiled in NetProAccess Point S7ONLINECP1623(ISO) or CP1623(RFC1006)NDIS / Local Adapter (Misconfigured)Must bind strictly to CP1623 in Configured ModeStation Configuration EditorSlot 1: WinCC, Slot 4: CP 1623CP 1623 missing or Stop stateSlot layout must match STEP 7 HW Config PC StationWinCC S7 Channel Driver LogStatus: 0x0000 (Connection Active)Error: 0xFFDF 410E (Connection Refused)Diagnose log in ..\WinCC\Diagnose\CP 1623 Operating ModeConfigured ModePG Operation / Manual ModeMust be set to Configured Mode via Station Configurator

Hardware notice: Verify CP 1623 firmware and SIMATIC NET Hardnet software entitlements with authorized Siemens distributors.

Configuring Symmetric S7 Named Connections in NetPro

To ensure both servers establish S7 connections symmetrically:

Verify NetPro Connection Table in SIMATIC Manager:
* Open NetPro on the Engineering Station.
* Select the S7-400 CPU. In the lower connection table, verify that two distinct S7 connections exist for each AS:
- Connection 1: Partner OS_Server_A (Connection Name: AS_to_OS_A).
- Connection 2: Partner OS_Server_B (Connection Name: AS_to_OS_B).
- Confirm that both connections use identical connection types (S7 connection or Fault-tolerant S7 connection for S7-400H).
Compile and Download Network Configurations:
* In NetPro, select Network > Save and Compile > Check and compile all.
* Download the network configuration to all AS controllers.
* In SIMATIC Manager, execute Compile OS for both Server A and Server B, selecting Scope: Entire OS to regenerate the WinCC connection descriptors (Named Connections).

Setting Up Station Configuration Editor and Set PG/PC Interface for CP 1623

On the failing Server B, configure local hardware drivers:

Configure Station Configuration Editor:
* On Server B, open the Station Configuration Editor from the Windows taskbar.
* Confirm that the Station Name matches the PC Station name in SIMATIC Manager exactly (case-sensitive).
* Verify slot allocations:
- Slot 1: WinCC Application (Status: OK).
- Slot 4 (or configured slot): CP 1623 (Status: OK, Mode: Configured).
- If the CP 1623 is missing or shows a red error, click Add, select CP 1623, and assign the slot matching HW Config.
Assign Access Point in Set PG/PC Interface:
* Open Windows Control Panel and launch Set PG/PC Interface.
* Under Access Point of the Application, select S7ONLINE.
* In the interface parameter assignment list, select:
CP1623(ISO) (for ISO transport) or CP1623(RFC1006) (for ISO-on-TCP).
* Never select CP1623(Auto) or a generic NDIS commercial network adapter.
* Click OK to save bindings.
Download Server Project from Engineering Station:
* From the ES, right-click the Server B OS project and execute PLC > Download.
* Start WinCC Runtime on Server B.

Verification

Verify that communication is fully established:
1. Open the diagnostic log on Server B:
C:\Program Files (x86)\Siemens\WinCC\Diagnose\SIMATIC_S7_PROTOCOL_SUITE_01.LOG
2. Confirm that connection entries report status: Connection established to AS_Unit1.
3. In WinCC Explorer, launch Channel Diagnosis:
* Confirm that all connections under Named Connections show green checkmarks with status OK.
4. Open process screens on Server B: Confirm that all process values display live numbers with zero grayed-out fields.

Limitations and Operational Precautions

Never change the Station Name in the Station Configuration Editor while WinCC Runtime is active; changing station names breaks SQL Server database security associations.
If using the CP 1623 for fault-tolerant connections (S7-REDCONNECT), ensure the S7-REDCONNECT software license is installed via Automation License Manager on both servers.
Both redundant OS Servers must be attached to the same physical Industrial Ethernet ring with identical SCALANCE switch VLAN configurations.

Engineer FAQ

What does error code 0xFFDF 410E in the S7 protocol log mean?

Error 0xFFDF 410E translates to 'Connection setup rejected by partner'. This indicates the target S7-400 CPU does not have a matching S7 connection configured in its online NetPro table for that server's TSAP/MAC address.

Can Server A and Server B have different CP 1623 slot numbers?

No. Redundant OS Server pairs must have identical hardware configurations (same slot assignments in Station Configuration Editor) to ensure seamless project synchronization.

Why does WinCC show disconnected even if the CP 1623 LEDs are green?

Green LEDs on the CP 1623 indicate Layer 1/2 physical link and ring redundancy. Application-layer S7 communication requires successful authentication of NetPro connection tables.