Resolving Redundant CPU 410-5H Link-Up Refusal and System Expansion Card (SEC) Mismatches

In a fault-tolerant SIMATIC PCS 7 CPU 410-5H redundant system, when Rack 0 is in RUN and Rack 1 remains stuck in STOP with the diagnostic buffer error 'Refusal of CONNECT due to different structure of the part AS' (Event ID 16#430E / 16#49A0), the standby CPU cannot synchronize because of an architectural discrepancy between the two subsystems. In CPU 410-5H systems, the primary cause is a mismatch between the System Expansion Cards (SEC): both CPUs must possess identical licensed Process Object (PO) capacities (e.g. both 500 PO). If one CPU was upgraded or replaced with an un-upgraded 100 PO card, link-up is refused. To resolve this: align the SEC capacities using the CPU 410 Expansion wizard, ensure identical firmware releases, and reset the standby CPU to factory settings.

Executive Summary

High-availability process automation architectures utilizing the redundant SIMATIC S7-400H / CPU 410-5H rely on strict hardware and structural symmetry between the primary and standby controllers. Following a hardware replacement or scheduled maintenance turnaround, control engineers frequently encounter a critical barrier where Rack 0 enters RUN, but Rack 1 remains stuck in internal STOP, refusing to transition to RUN despite healthy fiber-optic synchronization modules. Querying the diagnostic buffer reveals the event Refusal of CONNECT due to different structure of the part AS. This article analyzes the architectural synchronization prerequisites of CPU 410-5H controllers, explains why System Expansion Card (SEC) capacity discrepancies block link-up, and provides the step-by-step restoration procedure.

Architecture and Fundamental Mechanics

S7-400H Link-Up and Update Mechanics, CPU 410-5H Symmetry Constraints, and SEC Licensing Rules:

The S7-400H Redundant Synchronization Sequence (Link-Up and Update):
When the standby CPU initializes, it executes the Link-Up phase over the high-speed optical sync submodules.
The master CPU verifies whether the standby subsystem possesses identical hardware capabilities, memory geometry, and firmware revisions.
If verified, the system proceeds to the Update phase, copying the active work memory, timers, and I/O states to the standby CPU.
If ANY architectural asymmetry is detected, the master aborts link-up and commands the standby into internal STOP, recording Event ID 16#430E or 16#49A0 (Refusal of CONNECT due to different structure of the part AS).
Root Cause 1: System Expansion Card (SEC) PO Capacity Mismatch:
In classical S7-400H CPUs (e.g. CPU 414-5H), memory is dictated by RAM cards.
In the CPU 410-5H, technological memory and execution limits are governed by the hardware System Expansion Card (SEC) plugged into the rear of each CPU module.
The Redundancy Symmetry Rule: Both CPUs in an H-pair MUST have System Expansion Cards with the exact same licensed PO capacity (e.g. both 100 PO, both 500 PO, or both 1600 PO).
If Rack 0 has a 500 PO SEC (upgraded via an SEC PO expansion pack) and Rack 1 is replaced with a spare CPU containing a factory default 100 PO card, the master CPU detects incompatible structural capacity and refuses to connect.
Root Cause 2: Firmware Revision Asymmetry:
Both CPU 410-5H controllers must run the exact same firmware revision (e.g. V8.2.1).
A minor build mismatch between the two CPUs will cause refusal of connect during the handshake.
Root Cause 3: Inconsistent Memory Configuration or Hardware Residuals:
If the replacement CPU retained residual configuration blocks or partition assignments from a previous single-controller project, it fails to negotiate the redundant memory layout until a factory reset is executed.

Step-by-Step Implementation Procedure

Inspect the Diagnostic Buffer of the Standby CPU:
* In SIMATIC Manager, select the standby CPU in Rack 1 and press Ctrl + D.
* Verify the diagnostic event: Event ID 16#430E - 'Refusal of CONNECT due to different structure of the part AS'.
* Check previous event lines to confirm whether the mismatch references memory capacity, firmware, or hardware modules.
Verify and Align System Expansion Card (SEC) Capacities:
* In SIMATIC Manager, select the primary CPU in Rack 0 -> PLC -> Display Available Technical Capabilities.
* Note the PO capacity (e.g. 500 PO).
* Select the standby CPU in Rack 1 -> PLC -> Display Available Technical Capabilities.
* If Rack 1 indicates a lower capacity (e.g. 100 PO):
- An SEC PO Upgrade must be applied to Rack 1.
- Note: In redundant CPU 410-5H systems, CPU 410 PO Upgrade packages must be procured and applied to BOTH CPUs.
- In SIMATIC Manager, execute PLC -> Upgrade Technical Capabilities... on Rack 1 to transfer the upgrade entitlement into its SEC.
Reset the Standby CPU 410-5H to Factory Settings (Section 9.8 of CPU 410 Manual):
* If residual memory configurations persist on the replacement CPU, perform a hardware factory reset:
1. Switch off line voltage (power supply) to Rack 1.
2. Press and hold down the Reset button on the CPU 410-5H front panel.
3. While holding the Reset button, switch the line voltage back on.
4. Wait until the LED pattern displays pattern 1 (INTF, EXTF, BUS1F, BUS2F flash together).
5. Release the Reset button, and within 3 seconds, press and hold it down again for 3 seconds.
6. The CPU clears all internal memory and reboots to factory delivery condition.
Verify Firmware Alignment:
* In SIMATIC Manager, check the firmware version on both CPUs (PLC -> Module Information -> General).
* If versions differ, execute an online firmware update on the standby CPU so that both modules run identical firmware.
Re-initiate Redundant Link-Up:
* Switch the standby CPU operating switch to RUN.
* The master CPU initiates Link-Up; verify that both fiber-optic sync LEDs illuminate steady green.
* The system completes Update, and Rack 1 smoothly enters RUN (Redundant) mode.

Verification

Inspect the front panel LEDs on both CPU 410-5H modules; verify that both CPUs show solid green RUN LEDs.
Open the Diagnostic Buffer on Rack 0; verify the entry: Event ID 16#4301 - 'Transition to REDUNDANT mode'.
In SIMATIC Manager, execute a test delta download; confirm that changes download synchronously to both CPUs in RUN mode.

Limitations and Operational Precautions

Never attempt to operate an S7-400H / CPU 410-5H pair with asymmetric SEC cards; the operating system strictly forbids non-redundant capacity differences. Procuring CPU 410 PO Upgrades requires purchasing separate upgrade licenses for each CPU in the redundant pair. Validate final license quantities and MLFBs against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

What causes 'Refusal of CONNECT due to different structure of the part AS' on CPU 410-5H?

It is caused by architectural asymmetries between the redundant CPUs, most commonly mismatched System Expansion Card (SEC) PO capacities or differing firmware versions.

Must both CPUs in a CPU 410-5H redundant pair have identical SEC cards?

Yes. Both CPUs must have System Expansion Cards with the exact same licensed Process Object capacity.

If I upgrade POs on a redundant CPU 410-5H system, do I need two upgrade packages?

Yes. Redundant CPU 410-5H controllers require purchasing and applying matching PO upgrades to both physical CPUs.

How do you reset a CPU 410-5H to factory settings?

Power off the CPU, hold the Reset button while powering on, release when the LED pattern appears, and press and hold again for 3 seconds.

Do the fiber-optic sync modules have to be identical?

Yes. Both synchronization submodules and fiber-optic patch cables must be identical in length, speed, and catalog specification.

Technical References

CPU 410-5H

Refusal of CONNECT

System Expansion Card

SEC Mismatch

S7-400H

Redundancy Synchronization

Factory Reset

PCS 7 V9.0

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Resolving Redundant CPU 410-5H Link-Up Refusal and System Expansion Card (SEC) Mismatches