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Integrating Redundant S7-400H / CPU 410-5H with Non-Redundant S7-300, S7-1200, and S7-1500 PLCs

Integrating Redundant S7-400H / CPU 410-5H with Non-Redundant S7-300, S7-1200, and S7-1500 PLCs

In SIMATIC PCS 7, interfacing a fault-tolerant S7-400H or CPU 410-5H controller with non-redundant partner PLCs (such as S7-300, S7-1200, or S7-1500) over Industrial Ethernet cannot utilize native bumpless S7-REDCONNECT because the non-redundant controllers lack dual network interfaces and redundancy stacks. The certified architectures are: configuring two one-way S7 connections from the S7-400H using standard PUT/GET (SFB 14/15) or Industry Library S7Put/S7Get blocks, deploying Open User Communication (Modbus TCP) for vendor-neutral interoperability, or utilizing hardware couplers like the DP/DP Coupler or Y-Link for deterministic PROFIBUS integration.

Table of Contents

The Asymmetric Redundancy Challenge in Industrial Ethernet

In modern process plants, the core plant is often controlled by a high-availability S7-400H or CPU 410-5H system running SIMATIC PCS 7, while auxiliary package units, skids, and vendor packages utilize standalone, non-redundant controllers (such as S7-300, S7-1200, S7-1500, or third-party PLCs).

Engineers routinely face the challenge of establishing bidirectional data exchange (interlocks, start/stop permissives, analog setpoints, and production totals) between these asymmetric systems:
* The S7-400H features dual redundant CPU sub-racks (Rack 0 and Rack 1) and dual Industrial Ethernet interfaces (CP 443-1 or CPU onboard PN ports).
* The S7-300 / S7-1200 / S7-1500 possesses only a single CPU and a single network interface.

Because standard S7-REDCONNECT fault-tolerant connections require redundant hardware at both ends of the communication path, S7-REDCONNECT cannot be established directly to a standalone PLC. A specialized engineering architecture must bridge the redundancy gap.

Communication Options - S7 Protocol vs Modbus TCP vs Hardware Couplers

Siemens provides four verified engineering methodologies to bridge redundant S7-400H systems to non-redundant controllers:

1. One-Way S7 Connections via PUT/GET (Client Mode on S7-400H)

  • The S7-400H acts as the active communication Client, while the S7-300/1200/1500 acts as a passive Server.
  • In NetPro, configure two unspecified S7 connections pointing to the single IP address of the remote PLC: Connection 1 originating from CPU Rack 0, and Connection 2 originating from CPU Rack 1.
  • In CFC, standard system function blocks SFB 14 (GET) and SFB 15 (PUT) query and write data. Whichever S7-400H CPU is currently the active Master executes the communication calls.
  • TIA Portal Requirement: In S7-1200/1500 CPU properties, Permit access with PUT/GET communication from remote partner must be checked, and target DBs must have 'Optimized block access' unchecked.

2. SIMATIC PCS 7 Industry Library (S7Put / S7Get)

  • Part of the optional PCS 7 Industry Library (IL).
  • Replaces raw SFB 14/15 with technological blocks: S7Get (FB 1108) and S7Put (FB 1109).
  • Includes automatic lifebeat monitoring, communication timeout detection, and native WinCC alarm generation (ALARM_8P) if the remote PLC fails.

3. Open User Communication / Modbus TCP

  • Operates over standard TCP/IP.
  • The S7-400H runs the Open Modbus/TCP block package (MODBUSPN / MODBUS_H).
  • Ideal for heterogeneous multi-vendor plants where communications must interface seamlessly with both Siemens S7-1500 and third-party controllers (e.g., Allen-Bradley ControlLogix).

4. Hardware Couplers (PROFIBUS DP with Y-Link or DP/DP Coupler)

  • If physical determinism and electrical isolation are required, a DP/DP Coupler (6ES7158-0AD01-0XA0) or Y-Link bridges the redundant S7-400H master bus to the non-redundant slave.

Comparison Table - Inter-Controller Communication Architectures

The following table compares the four integration architectures across engineering effort, hardware requirements, and diagnostic capabilities.

ArchitectureProtocolNetwork MediaRedundancy SwitchingDiagnostic IntegrationAdditional Software / HardwareStandard S7 PUT/GETS7 ProtocolIndustrial EthernetHandled in S7-400H logicBasic (NDR/Error bits)None (Standard STEP 7 SFBs)Industry Library S7Put/S7GetS7 ProtocolIndustrial EthernetNative IL block logicFull APL Alarms & FaceplatesPCS 7 Industry Library LicenseOpen Modbus/TCPModbus TCPIndustrial EthernetRedundant H-libraryStandard Modbus statusModbus/TCP Redundant LicenseDP/DP Coupler / Y-LinkPROFIBUS DPRS-485 Copper / FiberHardware bumplessHardware diagnostic interruptsPhysical Coupler Module

Step-by-Step Configuration for S7 PUT/GET Communication

Follow this verified engineering workflow to connect an S7-400H / CPU 410-5H to a non-redundant S7-1500 PLC using S7 communication:

  1. Configure S7-1500 in TIA Portal:
    * Open the S7-1500 project in TIA Portal.
    * Open CPU Properties -> Protection & Security -> Connection mechanisms.
    * Check Permit access with PUT/GET communication from remote partner.
    * In the target Data Block (e.g., DB100), open properties and uncheck Optimized block access (use standard S7-300/400 DB structure).
    * Download to the S7-1500.

  2. Configure Unspecified S7 Connections in NetPro (PCS 7):
    * In SIMATIC Manager, open NetPro.
    * Select CPU 410-5H (Rack 0). In the connection table, insert a new S7 connection.
    * Partner: Unspecified.
    * In connection properties:

    • Enter the static IP address of the S7-1500.
    • Partner TSAP / Rack: Rack 0, Slot 1.
    • Note the assigned Local ID (e.g., W#16#1001).
    • Repeat the identical configuration for CPU 410-5H (Rack 1), generating Local ID W#16#1002.
    • Save and compile NetPro.

  3. Implement Communication in CFC:
    * In CFC, insert SFB 14 (GET) and SFB 15 (PUT) or Industry Library S7Get / S7Put.
    * Interconnect the local connection ID (ID) dynamically to point to the active CPU master connection.
    * Configure the remote DB pointer (e.g., P#DB100.DBX0.0 BYTE 100).
    * Trigger communication cyclically (e.g., every 500 ms in OB33) using a clock pulse generator.

  4. Implement Heartbeat / Lifebeat Watchdog:
    * Inside the exchanged DB, configure a heartbeat integer counter that the S7-1500 increments every 100 ms.
    * In the S7-400H, monitor the counter. If the value does not change within 3 seconds, declare a communication fault and force safe interlock positions.

Verification

Confirm that inter-controller communication is robust and fault-tolerant:

  1. Verify Live Data Exchange:
    * Open the shared DB in CFC Test Mode on the CPU 410-5H and online in TIA Portal on the S7-1500.
    * Change a test setpoint in S7-1500. Confirm that the CPU 410-5H updates within 500 ms.
  2. Test S7-400H Controller Redundancy Failover:
    * Power down or switch the Master CPU of the 410-5H to STOP.
    * Confirm that the Standby CPU becomes Master bumplessly.
    * Confirm that data exchange with the S7-1500 continues uninterrupted over the secondary S7 connection.
  3. Test Remote PLC Disconnection:
    * Disconnect the Ethernet cable from the S7-1500.
    * Confirm that the S7-400H watchdog logic detects the frozen heartbeat, trips communication alarms, and asserts safe default values.

Limitations and Operational Precautions

  • Unprotected Remote DB Overwrites: PUT commands write directly into remote PLC memory without remote CPU program mediation. Ensure memory ranges do not conflict with local logic in the remote PLC.
  • TIA Portal Security Enforcement: Firmware V4.0+ on S7-1200 and Firmware V1.8+ on S7-1500 enforce PUT/GET access restrictions by default. Forgetting to enable PUT/GET in TIA Portal results in permanent communication errors (Error = 1, Status = 16#0001).
  • Connection Resource Limits: Standard S7-1200 PLCs have strictly limited concurrent S7 connection resources (typically 2 to 4 connections max). Avoid multiple engineering systems connecting simultaneously.

Engineer FAQ

Can S7-1200 initiate PUT/GET to an S7-400H?

While possible, configuring the non-redundant PLC as the client is an antipattern because the standalone PLC cannot easily detect which S7-400H CPU is active Master, leading to connection failures during H-system failovers. The S7-400H should always be the active Client.

What is the maximum data block size that can be transferred in a single PUT/GET call?

Standard S7 communication limits a single PUT/GET telegram to 160 to 400 bytes depending on the communications processor and PDU size. Larger data sets must be partitioned across multiple calls.

Does Industry Library S7Get/S7Put require separate runtime licenses?

Yes. The PCS 7 Industry Library is an optional software package requiring engineering and runtime licensing managed via Automation License Manager.

Can OpenPCS 7 be used instead of controller-to-controller communication?

OpenPCS 7 is designed for PC-to-controller SCADA data access over OPC. For real-time deterministic interlocking between controllers, direct controller-to-controller communication (S7 or Modbus TCP) is mandatory.

Technical References

Related Articles

Related Tags

  • CPU410-5H
  • S7-400H
  • S7-1500
  • S7-1200
  • PUT-GET
  • Industry-Library
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