Configuring Fault-Tolerant S7 Connections with CP 443-1 RNA and NetPro Redundancy Constraints in PCS 7

In SIMATIC PCS 7, configuring 4-way maximum cross-redundant S7 connections (Rack 0/1 to Rack 0/1) in NetPro is restricted when using CP 443-1 RNA modules (6GK7443-1RN00-0XE0). CP 443-1 RNA is an industrial Ethernet processor designed specifically for Layer 2 redundancy via Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR), listed under the 'Standard' hardware catalog rather than the dedicated 'PCS 7' profile. In STEP 7 V5.6 HF4 and PCS 7 V9.0 SP3, NetPro connection routing for RNA modules enforces protocol-level PRP handling rather than multi-path S7 fault-tolerant connection meshes. For standard S7 fault-tolerant 4-way AS-to-AS or AS-to-OS communication without external PRP switches, use the standard PCS 7 communications processor CP 443-1 (6GK7443-1EX30-0XE0).

Executive Summary

During control system modernization from legacy SIMATIC PCS 7 V7.0 to PCS 7 V9.0, system integrators often attempt to upgrade communications hardware to CP 443-1 RNA (6GK7443-1RN00-0XE0) to capitalize on bumpless Layer 2 network redundancy governed by the Parallel Redundancy Protocol (PRP / IEC 62439-3). However, when configuring fault-tolerant S7 connections in NetPro between redundant S7-400H controller racks, engineers discover that the 'Maximum Redundancy' option (establishing 4 cross-coupled subconnections: Rack 0->Rack 0, Rack 0->Rack 1, Rack 1->Rack 0, Rack 1->Rack 1) is greyed out or unsupported. This article explains the architectural distinction between Layer 2 hardware network redundancy (PRP) and Layer 4 S7 fault-tolerant software redundancy, analyzes STEP 7 NetPro routing restrictions, and outlines standard engineering solutions.

Architecture and Fundamental Mechanics

Layer 2 Hardware Redundancy (PRP) vs Layer 4 S7 Protocol Redundancy (S7-REDCONNECT):

Layer 4 S7-400H Redundant Communication Mesh:
In traditional SIMATIC PCS 7 systems using standard CP 443-1 modules (6GK7443-1EX30-0XE0), fault tolerance is managed at the transport layer by S7-REDCONNECT software protocols.
NetPro generates a 4-way cross-redundant connection mesh between two S7-400H stations. If one physical network cable, switch, or CP card fails, the S7-REDCONNECT stack detects the link timeout and automatically switches telegram transmission to one of the three surviving subconnection routes.
Layer 2 PRP Architecture with CP 443-1 RNA:
The CP 443-1 RNA (Redundant Network Access) module implements Parallel Redundancy Protocol (IEC 62439-3) directly in hardware ASIC logic.
The card has two physical Ethernet ports (Port A and Port B) that connect to two completely separate, parallel Ethernet networks (LAN A and LAN B). For every transmitted telegram, the RNA hardware duplicates the packet and transmits it across both ports simultaneously. The receiving RNA module accepts the first packet to arrive and immediately discards the duplicate with zero failover time.
Because redundancy is managed invisibly at Layer 2, the higher-level operating system and NetPro view the CP 443-1 RNA as a single network interface with a single MAC/IP identity, rendering multi-path Layer 4 connection mesh routing technically incompatible.
Catalog Profile Filtering and STEP 7 Changes:
CP 443-1 RNA was released for standard SIMATIC S7-400 automation and is categorized under 'Standard' in HW Config. It is excluded from the certified 'PCS 7' catalog profile.
Starting in STEP 7 V5.6 HF4 (delivered with PCS 7 V9.0 SP3), Siemens enforced strict NetPro connection rules that blocked 4-way cross-redundancy selection on RNA hardware to prevent conflicting failover mechanisms.

Step-by-Step Implementation Procedure

Determine Plant Network Redundancy Architecture:
* Evaluate whether the physical plant bus is engineered with dedicated PRP network architecture (e.g. SCALANCE X-204RNA network switches and dual disjoint rings) or standard industrial Ethernet switches.
* If the plant bus is a standard dual-bus PCS 7 architecture without PRP switches, replace CP 443-1 RNA with the released PCS 7 standard communication processor CP 443-1 (6GK7443-1EX30-0XE0).
Configuring Standard CP 443-1 EX30 for 4-Way Maximum Redundancy:
* In SIMATIC Manager, open HW Config for both S7-400H subracks.
* Set Catalog Profile to PCS7_V90.
* Insert CP 443-1 (6GK7443-1EX30-0XE0) into Rack 0 and Rack 1 of both AS stations.
* Assign IP addresses to Subnet A (Rack 0) and Subnet B (Rack 1).
* Save and Compile HW Config for all stations.
Configure S7 Fault-Tolerant Connection in NetPro:
* Open NetPro from SIMATIC Manager.
* Select CPU 0 in AS1, right-click and choose 'Insert New Connection'.
* Select target CPU 0 in AS2 (or the WinCC OS Application).
* Set Connection Type to 'S7 fault-tolerant connection'.
* Open Connection Properties and navigate to the 'Redundancy' tab.
* Select 'Maximum redundancy'. Confirm that NetPro creates 4 physical subconnection paths.
* Select Network -> Save and Compile ('Compile and check everything').
Alternative: Configuring CP 443-1 RNA under Layer 2 PRP Mode:
* If CP 443-1 RNA must be retained for compliance with plant-wide PRP standards:
* Switch HW Config profile to 'Standard'.
* In NetPro, configure standard S7 connections rather than 4-way fault-tolerant meshes.
* Connect Port A to LAN A and Port B to LAN B of the PRP network. The RNA hardware provides bumpless failover transparently to NetPro.
Download Connection Tables to Controllers:
* In NetPro, select PLC -> Download to Current Project -> Selected and Partner Stations.
* Perform connection table download during a planned maintenance window.

Verification

Verify Redundant Communication Operation:
1. In NetPro, verify that 'Compile and check everything' completes with 0 errors and 0 warnings.
2. In SIMATIC Manager, open Module Information for CPU 0 and CPU 1; verify that the S7 fault-tolerant connection reports status 'Established' and 'Active'.
3. Disconnect the primary Ethernet cable from Rack 0 CP. Confirm that process data communication continues without interruption or controller stop.
4. Reconnect the cable and verify automatic diagnostic clearing in the AS diagnostic buffer.

Limitations and Operational Precautions

Operational Precautions and Compatibility Boundaries:
- CP 443-1 RNA (6GK7443-1RN00-0XE0) is not listed in the certified PCS 7 Hardware Catalog profile; utilizing it requires bypassing catalog filters, which may complicate official Siemens technical support assistance.
- Never connect both ports of a CP 443-1 RNA module into standard unmanaged commercial switches; PRP frame duplication will cause broadcast storms if networks are not properly segregated.
- Always verify that OS servers communicating with S7-400H stations have compatible industrial Ethernet cards (e.g. CP 1623 / CP 1628) and valid SIMATIC NET S7-REDCONNECT licenses.

Engineer FAQ

Why is 'Maximum Redundancy' greyed out with CP 443-1 RNA in NetPro?

CP 443-1 RNA manages redundancy at Layer 2 (PRP hardware frame duplication). NetPro does not permit 4-way Layer 4 connection meshes on interfaces that already duplicate frames at Layer 2.

What is the officially recommended CP for PCS 7 S7-400H systems?

The officially certified and released communications processor for PCS 7 V8.x and V9.x is CP 443-1 (order number 6GK7443-1EX30-0XE0).

Can CP 443-1 RNA communicate with standard WinCC OS servers?

Yes, CP 443-1 RNA can communicate with WinCC, but bumpless PRP redundancy requires OS stations to be equipped with PRP-capable network cards (such as CP 1628) or connect through external SCALANCE X-204RNA switches.

What happened in STEP 7 V5.6 HF4 regarding NetPro connections?

Starting in STEP 7 V5.6 HF4, Siemens updated NetPro routing validation rules, restricting multi-path S7 fault-tolerant connection generation across non-standard communication processors.

Does CP 443-1 RNA support bumpless failover?

Yes. When connected to a properly segregated PRP network (LAN A and LAN B), CP 443-1 RNA provides true zero-millisecond bumpless packet failover per IEC 62439-3.