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Configuring Redundant Plant Bus Connections Across Fiber and Routed IP Wireless Networks in S7-400H Systems

Configuring Redundant Plant Bus Connections Across Fiber and Routed IP Wireless Networks in S7-400H Systems

In SIMATIC PCS 7 S7-400H systems, an S7 Fault-Tolerant Connection configured in NetPro cannot directly mix Layer 2 ISO protocol across the primary fiber network with Layer 3 routed IP addressing across a secondary 3G/wireless gateway, because pure ISO transport frames (MAC broadcasts) cannot cross IP routers. To implement automated, bumpless communication failover between fiber and wireless networks, standardize both physical networks on ISO-on-TCP (RFC 1006). In NetPro, configure dual Industrial Ethernet subnets with distinct IP ranges, assign static IP addresses to the CPs and gateways, and establish a single S7 Fault-Tolerant Connection between the WinCC Redundant Servers and the S7-400H CPUs across both subnets.

Table of Contents

Executive Summary

Continuous process facilities often operate remote pumping stations, wellheads, or offsite chemical storage units that require high-availability automation. When integrating remote S7-400H controllers with central redundant WinCC OS Servers, infrastructure designs frequently mandate dual communication media: a primary terrestrial fiber optic cable running high-speed Industrial Ethernet, and a secondary backup wireless link (utilizing industrial 3G/4G cellular routers or directional wireless bridges). However, when engineers attempt to configure the primary fiber link using pure Layer 2 ISO transport protocol (via CP 1613/1623) while attempting to address the wireless gateway via IP addressing, NetPro refuses compilation or fails to failover. Resolving this architecture requires understanding protocol boundaries and standardizing on ISO-on-TCP.

S7 Fault-Tolerant Connection Architecture in NetPro

An S7 Fault-Tolerant Connection (S7-Connection Fault-Tolerant) provides automatic communication failover for S7-400H:

  • Involves four potential communication paths between a redundant OS Server and an S7-400H CPU pair (Server NIC 1/2 to AS CP 1/2).
  • SIMATIC NET and WinCC continuously monitor heartbeat telegrams on all four paths.
  • If the active path breaks, the communication stack automatically switches to an alternate path within 100 to 300 ms without loss of data, alarms, or operator intervention.

The Protocol Conflict: Layer 2 ISO vs Layer 3 Routed IP Networks

The fundamental obstacle in hybrid fiber/wireless architectures stems from protocol mechanics:

  • Pure ISO Protocol (Layer 2): Identifies endpoints by MAC address. Operates with microsecond speed and high determinism, but cannot pass through standard IP routers or cellular 3G/4G VPN tunnels.
  • TCP/IP & Cellular Gateways (Layer 3): Routes packets across subnets and mobile networks using IP addresses. Routers strip Layer 2 headers, dropping pure ISO packets immediately.
  • NetPro Constraint: A single S7 Fault-Tolerant Connection requires all subnets to share a compatible protocol suite; you cannot have Path A operate on pure ISO and Path B operate on TCP/IP.

Standardizing on ISO-on-TCP (RFC 1006) for Mixed Media Networks

Siemens solves this protocol divergence using ISO-on-TCP (RFC 1006):

  • Encapsulates ISO transport headers inside standard TCP/IP packets on port 102.
  • Retains the deterministic framing and large block transfer capabilities of ISO protocol.
  • Traverses industrial routers, firewalls, and 3G/4G cellular gateways seamlessly.
  • Enables a unified S7 Fault-Tolerant Connection spanning both the local high-speed fiber switch and the routed wireless gateway.

Plant Bus Network Media and Redundancy Matrix

The following table compares network configurations for hybrid S7-400H communication paths:

Communication PathPhysical MediumProtocol UsedPort / TransportRouter / 3G Traversable?Primary Path (Local)Fiber Optic (SCALANCE)ISO-on-TCP (RFC 1006)TCP Port 102Yes (Layer 2 & Layer 3)Secondary Path (Backup)3G/4G Gateway / VPNISO-on-TCP (RFC 1006)TCP Port 102Yes (Full IP Routing)Invalid Mixed ConfigFiber (Pure ISO) + 3G (IP)Incompatible ProtocolsMAC vs IPNo (NetPro Compile Error)Hardwired Ring (MRP)Fiber Optic CablePure ISO or ISO-on-TCPRing TopologyLocal ring redundancy only.

Step-by-Step Procedure to Configure Dual-Subnet Fault-Tolerant Connections

Method: Configuring NetPro and Hardware Configuration

  1. In HW Config, open the S7-400H station. For each CP 443-1 in Rack 0 and Rack 1:
    * Enable IP protocol is used.
    * Assign static IP addresses (e.g. 192.168.10.11 for Rack 0, 192.168.10.12 for Rack 1).
    * Set the Gateway IP address to the industrial cellular router's local gateway.
  2. Open NetPro in SIMATIC Manager.
  3. Create two distinct Industrial Ethernet networks:
    * Network 1: Plant_Bus_Fiber (Subnet 192.168.10.0/24).
    * Network 2: Plant_Bus_Wireless (Subnet 10.20.30.0/24).
  4. Connect CP 443-1 ports and OS Server CP 1613/1623 adapters to both subnets.
  5. In NetPro, select the WinCC Application object under the OS Server station.
  6. Insert a new connection: Select target CPU S7-400H, and set Connection Type to S7 connection fault-tolerant.
  7. In connection properties, select ISO-on-TCP protocol.
  8. Under the Subnets/Interfaces assignment, check both the fiber and wireless subnets to enable dual-path routing.
  9. Execute Network > Save and Compile; confirm that NetPro compiles with 0 errors.
  10. Download NetPro connections to both the S7-400H CPUs and the WinCC OS Servers.

Verification

To verify automated communication failover:

  1. Baseline Connection Check: In WinCC Channel Diagnosis (WinCC Explorer > Channel Diagnosis), confirm that the S7 Protocol Suite driver shows status 'Ready' with active S7-Connection Fault-Tolerant.
  2. Primary Fiber Disconnection Test: Physically disconnect the primary fiber optic patch cable. Monitor WinCC Runtime.
  3. Bumpless Failover Audit: Verify that process tags continue updating smoothly via the 3G wireless gateway without process value freezes or WinCC connection loss alarms.
  4. Reconnection Recovery: Reconnect the fiber cable. Confirm that the connection stack automatically restores the primary high-speed path.

Limitations and Operational Precautions

  • Cellular Bandwidth & Latency: 3G/4G wireless connections exhibit variable latencies (50 to 300 ms); configure the S7 connection monitoring timeout in NetPro with a sufficient safety margin (e.g. 5 to 10 seconds) to prevent false failover cycling.
  • Data Plan Quotas: Cyclic WinCC polling over 3G cellular networks consumes substantial data bandwidth; optimize tag update cycles or configure event-driven polling.
  • Cybersecurity (IPsec VPN): Never route industrial control protocols across public cellular networks unencrypted; deploy industrial security routers (such as SCALANCE M874) with hardware IPsec or OpenVPN tunneling.

Engineer FAQ

Can standard commercial office network cards (NICs) support S7 Fault-Tolerant connections?

Yes, when paired with the SIMATIC NET SOFTNET-IE S7 software package, standard industrial Ethernet NICs support fault-tolerant S7 connections.

Does NetPro allow setting path priority between fiber and wireless?

While all paths are actively monitored, the primary path is typically preferred; bandwidth-constrained backup paths become active only upon primary loss.

Why did pure ISO protocol fail over the cellular gateway?

Pure ISO operates strictly at Layer 2 (MAC level) without IP headers; cellular gateways route Layer 3 packets and discard Layer 2 broadcasts.

Technical References

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Related Tags

  • https://industrialautomation.net/search?query=simatic-pcs7
  • https://industrialautomation.net/search?query=s7-400h
  • https://industrialautomation.net/search?query=netpro
  • https://industrialautomation.net/search?query=networking
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