S7-400H Redundant Connection Repair in NetPro: Resolving 'Local ID:1 The H-Connection Has to be Repaired'

In SIMATIC PCS 7 and STEP 7 NetPro, error 'Local ID:1; The H-connection has to be repaired. Please check redundant partner CPU on which the connection was created' occurs when an S7-400H CPU or CP card is replaced or re-addressed, breaking the bilateral four-path redundant link table. NetPro flags the connection line black, indicating an orphaned partner endpoint. To repair the connection, open NetPro, double-click the faulted H-connection in the connection table, click 'Repair' or re-select the redundant partner CPU and CP interfaces, execute 'Network -> Save and Compile' with 'Compile everything', and download connections to both redundant CPUs.

Executive Summary

During hardware maintenance, rack upgrades, or CPU replacements on fault-tolerant SIMATIC S7-400H systems, engineers frequently encounter an error in NetPro stating that the fault-tolerant S7 connection (H-connection) must be repaired. In the graphical network view, the connection line changes from solid color to black, and the connection table displays error status. This issue arises because S7-400H fault-tolerant connections maintain four underlying redundant communication sub-paths across both CPU racks. Replacing a hardware module or altering rack slot numbers breaks the internal link table. This guide provides the exact restoration sequence to repair broken H-connections and re-establish redundant communication.

Architecture and Fundamental Mechanics

The architecture of S7-400H fault-tolerant S7 connections (H-connections) relies on synchronized multi-path topologies:
1. Four-Path Redundancy Structure:
A complete H-connection between an S7-400H station (Rack 0 and Rack 1) and a redundant partner (such as a WinCC OS Server with CP 1623 or a partner S7-400H) comprises up to four independent physical sub-paths:
- Sub-path 1: CPU 0 / CP 0 $\rightarrow$ Partner CP 0
- Sub-path 2: CPU 0 / CP 0 $\rightarrow$ Partner CP 1
- Sub-path 3: CPU 1 / CP 1 $\rightarrow$ Partner CP 0
- Sub-path 4: CPU 1 / CP 1 $\rightarrow$ Partner CP 1
2. Why the Connection Breaks During Hardware Changes:
In NetPro, the master connection record is bound to the specific hardware IDs and slot positions of the CPUs and CPs declared in HW Config. When an engineer replaces a CPU, renames a rack, or changes a CP subnet assignment, NetPro detects that one or more sub-paths reference an invalid or altered object ID. NetPro flags the connection as broken ('Black') and disables compilation.
3. The NetPro Repair Routine:
The 'Repair' function parses the updated hardware configuration, interrogates the partner CPU in Rack 1, verifies that identical subnets and IP/MAC configurations exist on both sub-racks, and rebuilds the internal connection descriptor tables.

Step-by-Step Implementation Procedure

Inspect Connection Status in NetPro:
a. Open SIMATIC Manager and double-click the 'Networks' object or open NetPro.
b. Select the CPU in Rack 0: in the lower connection table, locate the faulted S7 connection (Local ID 1).
c. Note the warning: 'The H-connection has to be repaired'.
d. Observe that the redundant partner connection in Rack 1 is highlighted in black.
Execute the Connection Repair Function:
a. Double-click the faulted H-connection entry in the connection table to open its Object Properties.
b. Click the 'Special Properties' or 'Redundancy' tab.
c. If a 'Repair' button is displayed, click 'Repair'. NetPro automatically scans the partner rack and re-links the orphaned sub-paths.
d. If 'Repair' is unavailable, inspect the 'Connection Partner' drop-down list: manually re-select the partner CPU/CP in Rack 1.
e. Confirm that 'Enable max. CP redundancy (with 4 connection paths)' is checked if dual CPs are present on each rack.
f. Click OK to close the properties dialog.
Execute Global Network Compilation:
a. In NetPro, select Network -> Save and Compile.
b. Select 'Compile everything' to force NetPro to regenerate all routing tables, gateway descriptors, and cross-project links.
c. Confirm that compilation completes with 0 errors.
Download Updated Connection Tables to Controllers:
a. In NetPro, select the S7-400H station.
b. Execute PLC -> Download to Current Project -> Selected Station.
c. In the download scope dialog, select 'Connections and Gateways'.
d. Download to CPU 0, then download to CPU 1.
e. If communicating with WinCC OS servers, execute an OS compilation and download changes to the OS servers.

Verification

In NetPro online view, right-click the S7-400H CPU and select 'Display Connection Status'.
Verify that the H-connection reports status 'Established' with green health indicators across all redundant sub-paths.
In WinCC Explorer on the OS server, open 'Channel Diagnosis': verify that the S7 Named Connection shows 'OK' with 4 active paths.
Disconnect one Ethernet cable from CP 0: verify that communication continues bumplessly over CP 1 without connection dropouts.

Limitations and Operational Precautions

Modifying connection properties or compiling NetPro while the S7-400H system is running in standalone mode (single CPU running) can prevent synchronization; always ensure both redundant CPUs are in RUN-Redundant mode before downloading connection changes.

Engineer FAQ

Why did the connection turn black in NetPro?

A black connection indicates an orphaned or structurally broken S7 connection where one of the redundant partner endpoints is missing or has mismatched hardware parameters.

Can I just delete the connection and create a new one?

Yes. If the 'Repair' function fails to resolve the link table, deleting the corrupted connection in NetPro and creating a fresh 'S7 connection fault-tolerant' is standard practice.

Does downloading connections require an AS STOP?

In S7-400H systems, downloading 'Connections and Gateways' from NetPro can be performed in RUN mode without stopping the CPUs.

What does 4-path redundancy mean in an H-connection?

It means the communication can survive any combination of CPU, CP, switch, or cable failure by maintaining four independent transmission routes between partners.

Why must I select 'Compile everything' in NetPro?

Selecting 'Compile everything' forces NetPro to rebuild the entire global link database rather than relying on incremental delta cache files that may contain obsolete pointer references.