Why WinCC Named Connections Cannot Use Dual ISO and TCP Paths Simultaneously and How to Implement S7-REDCONNECT Fault-Tolerant Links

In SIMATIC PCS 7 and WinCC, it is fundamentally impossible to configure two independent Named Connections (one ISO Transport and one TCP/IP) to active process tags simultaneously for automated failover. When an Operator Station (OS) project is compiled, the AS-OS engineering compiler strictly binds all process tags to a single, specific Named Connection ID in the WinCC Tag Management database. If that connection fails, tags mapped to it cannot automatically hop to an alternate named connection. To achieve true redundant communication without losing process data, you must deploy fault-tolerant S7 connections using redundant S7-400H controllers, dual CP 443-1 communication processors, and the SIMATIC NET S7-REDCONNECT software package on the OS server.

Architecture of AS-OS Tag Compilation and Named Connections

In SIMATIC PCS 7, communication between automation stations and operator stations is governed by the Named Connection architecture:

Named Connection Definition: Configured in NetPro between the WinCC Application (on the OS PC Station) and the S7-400 CPU. A Named Connection abstracts physical IP addresses, MAC addresses, and backplane slot numbers behind a logical connection name (e.g., OS_AS1).
AS-OS Engineering Compilation: When an engineer compiles the OS (Compile OS), the engineering system parses all CFC block parameters flagged with S7_m_c := 'true'. It maps these variables directly under the designated Named Connection in WinCC Tag Management.
Tag Mapping Inflexibility: Every process tag in WinCC is hard-linked to its parent connection handle. WinCC does not support dynamic tag re-binding between different Named Connections at runtime.

Why Dual Named Connection Switching Is Architecturally Impossible

Process engineers occasionally attempt to achieve network redundancy by configuring two parallel Named Connections from the same CP 443-1 to WinCC—one using ISO Transport protocol and the second using TCP/IP:

The Fallacy: The assumption that if the TCP/IP stack encounters packet corruption, WinCC can seamlessly switch its process tags to the ISO connection.
Why It Fails:
1. The AS-OS compiler forces the engineer to choose one and only one connection path during compilation.
2. If an engineer manually created duplicate tags under the second connection, all Graphics Designer faceplates, trends, and alarm logging servers would still reference the original tag names on the primary connection.
3. WinCC Redundancy software is designed solely for server redundancy (synchronizing Master and Standby OS Servers), not for switching client tags between heterogeneous network protocol drivers.

Fault-Tolerant S7 Connections via SIMATIC NET S7-REDCONNECT

The official, industry-certified method for achieving communication redundancy between an S7-400 station and WinCC is the Fault-Tolerant S7 Connection powered by SIMATIC NET S7-REDCONNECT:

What Is S7-REDCONNECT?: A specialized communication protocol extension included with SIMATIC NET (requiring the SIMATIC NET S7-REDCONNECT runtime license). It enables the OS PC station to establish a 4-way redundant communication link with a fault-tolerant S7-400H controller.
How It Works:
The OS uses two network adapters (e.g., CP 1623 / CP 1613 or standard industrial NICs with Hardnet/Softnet).
The S7-400H utilizes two redundant CPUs and two independent CP 443-1 communications processors.
S7-REDCONNECT establishes four physical communication paths behind a single logical Named Connection.
If an Ethernet cable snaps, a network switch powers down, or a CP 443-1 fails, S7-REDCONNECT performs bumpless switching to an alternate physical path within milliseconds—completely transparent to WinCC and the operator faceplates.

Supported High-Availability Topologies: S7-400H and Dual CP 443-1

Siemens certifies three primary fault-tolerant communication topologies in the SIMATIC PCS 7 High Availability manual:

Redundant S7-400H with Dual CP 443-1 Processors (Maximum Availability):
* Two CPU 410-5H or CPU 417-4H controllers in Rack 0 and Rack 1.
* Two external CP 443-1 processors (6GK7443-1EX30-0XE0), one in each rack.
* Connected via dual redundant industrial switches (SCALANCE X) to redundant CP 1623 cards in the OS Server.
Single S7-400 Station with Dual CP 443-1 Cards:
* A single S7-400 CPU mounted in a central rack with two CP 443-1 processors.
* S7-REDCONNECT maintains redundant physical paths to protect against cable breaks and network card failures.
CPU 410-5H Integrated Redundant PROFINET Links:
* CPU 410-5H utilizing its dual integrated PN/IE ports connected to ring topologies or dual subnets, eliminating the need for external CP 443-1 modules.

Comparison of Standard vs Fault-Tolerant WinCC Communication Topologies

The table below compares standard single-path connections with redundant fault-tolerant communication architectures.

Architecture TypeSoftware License RequiredHardware Required (AS Side)Failover TimeImpact on WinCC TagsSingle Named ConnectionStandard SIMATIC NET1x CP 443-1 or Onboard PNNone (System halts on failure)All tags transition to Bad QualityDual Named Connections (Manual)Standard SIMATIC NET1x or 2x CP 443-1Not supportedTags cannot switch connectionsFault-Tolerant S7 (Single AS)S7-REDCONNECT2x CP 443-1 in single rack< 100 msBumpless; zero tag disruptionFault-Tolerant S7 (S7-400H)S7-REDCONNECTRedundant H-CPU + 2x CP 443-1< 30 msCompletely transparent to WinCCPROFINET System Redundancy (S2)Native PCS 7 V8.2+CPU 410-5H dual PN ports< 50 msManaged by PROFINET stack

Verification

To verify that a fault-tolerant S7 connection is operating correctly:
1. In NetPro on the ES, verify that the connection between the OS WinCC Application and the S7-400H is styled as a Fault-Tolerant S7 Connection (indicated by a double-line icon).
2. Download connection tables to AS and OS stations.
3. On the OS Server, launch SIMATIC NET > Station Configuration Editor. Confirm that the CP 1623 / IE General cards and S7-REDCONNECT application report RUN (green).
4. Open WinCC Channel Diagnosis: under SIMATIC S7 Protocol Suite > Named Connections, confirm that the connection state is Operational with error count 0.
5. Disconnect one Ethernet cable from CP 443-1 Rack 0: verify in WinCC Channel Diagnosis that the connection seamlessly shifts to Rack 1 without any tags showing bad quality or # symbols on the operator screens.

Limitations and Operational Precautions

Never attempt to configure redundant communication by creating duplicate tag databases in WinCC; doing so doubles tag licensing consumption and causes database desynchronization.
S7-REDCONNECT requires dedicated runtime licensing; deploying fault-tolerant connections without an active REDCONNECT license key will trigger recurring ALM license violation messages.
When configuring industrial switches for redundant plant buses, ensure that Spanning Tree (RSTP) or MRP ring convergence times are tuned below 200 ms to prevent redundancy watchdog timeouts.
Verify that hardware equipment schedules and software licensing align with the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

Can I switch between two named connections using a C-Action or VBScript in WinCC?

While a script could theoretically re-point internal tag references, doing so across thousands of process tags in runtime is completely impractical, causes severe CPU spikes, and cannot dynamically update Alarm Logging or Trend archives.

What is the difference between WinCC Redundancy and S7-REDCONNECT?

WinCC Redundancy synchronizes two OS Servers (Master and Standby) over a dedicated network link. S7-REDCONNECT provides redundant communication between an OS Server and an S7-400H controller over the Plant Bus.

Does CPU 410-5H require CP 443-1 for fault-tolerant connections?

No. The CPU 410-5H features integrated 2x 2-port PROFINET/Industrial Ethernet interfaces that natively support fault-tolerant S7 connections without requiring external CP 443-1 cards.