Establishing Fault-Tolerant S7 Connections Between CPU 410-5H and CPU 414-5H

Establishing a fault-tolerant S7 connection between a SIMATIC PCS 7 CPU 410-5H and a classic STEP 7 CPU 414-5H without communication loss requires configuring the connection in NetPro across a common Multiproject and ensuring subnets and IP gateways are aligned. Because CPU 414-5H provides only one onboard PROFINET interface (or requires an external CP 443-1 for redundant network routing) while CPU 410-5H provides dual interfaces, the fault-tolerant connection must be compiled and downloaded to both stations simultaneously via 'Selected Stations' or 'Connections and Gateways'. Runtime data exchange is executed using BSEND (SFB12) and BRCV (SFB13) with matching R_ID parameters.

S7 Fault-Tolerant Connection Topology: CPU 410-5H vs CPU 414-5H

Connecting modern SIMATIC PCS 7 controllers (CPU 410-5H) with legacy or standalone manufacturing lines running S7-400H controllers (such as CPU 414-5H) requires establishing a high-availability S7 Fault-Tolerant Connection. A fault-tolerant connection establishes up to four underlying communication pathways (two controllers $\times$ two network subnets), enabling bumpless data exchange even if one controller or one physical network switch fails.

Engineers frequently struggle with this integration when the two systems are engineered in separate projects. S7 fault-tolerant connections cannot be negotiated dynamically as raw TCP sockets; both endpoints must be defined inside NetPro to generate the internal communication job tables.

Interface Architecture and External CP 443-1 Requirements

A vital hardware consideration differentiates CPU 410-5H from CPU 414-5H:
- CPU 410-5H: Features dual onboard PROFINET interfaces (Interface 1 and Interface 2, total 4 physical ports), supporting redundant plant bus connections natively without extra CP cards.
- CPU 414-5H: Possesses only a single onboard PROFINET interface (plus PROFIBUS DP). To achieve genuine 4-way media and controller redundancy across two separate Industrial Ethernet rings, the CPU 414-5H rack must be equipped with an external CP 443-1 (6GK7443-1EX30-0XE0).

If both stations share only a single flat Industrial Ethernet ring, a 2-way fault-tolerant connection can be configured using the onboard ports, provided both controller pairs share identical IP subnets.

Communication Block Configuration: BSEND and BRCV (SFB12/13)

The table below details communication function blocks, parameters, and roles for S7 fault-tolerant data exchange:

Block / ParameterType / DirectionValue / RangeFunctional DescriptionSFB12 (BSEND)System Function BlockLocal ControllerTransmits block-oriented data up to 64 KB across S7 connections.SFB13 (BRCV)System Function BlockPartner ControllerReceives data sent by BSEND into a designated data block (DB).IDWORD Inpute.g., 16#0001Connection ID assigned automatically in NetPro connection properties.R_IDDWORD InputUnique 16#...Coordination ID; must match identically on both BSEND and BRCV.DONE / NDRBOOL OutputPulseHandshake flags: DONE indicates send complete; NDR indicates new data received.ERROR / STATUSOutputDynamic CodeDiagnostic words: STATUS = 16#0000 indicates healthy transmission.

Step-by-Step NetPro Configuration and Download Procedure

Combine Projects into Common Multiproject:
- In SIMATIC Manager, select File -> New -> Multiproject.
- Add both the PCS 7 project (containing CPU 410-5H) and the STEP 7 project (containing CPU 414-5H) into this multiproject.
Harmonize Industrial Ethernet Subnet in NetPro:
- Open NetPro from SIMATIC Manager.
- Merge the Industrial Ethernet subnets of both projects so both controllers connect to the same logical subnet object (e.g., PlantBus_1).
Insert S7 Fault-Tolerant Connection:
- Select the CPU 410-5H controller in the NetPro grid.
- In the connection table, insert a new connection: select the CPU 414-5H as the partner and choose S7 fault-tolerant connection.
- Note the assigned Connection ID (e.g., W#16#1).
Program BSEND and BRCV in CFC or STL:
- In the CPU 410-5H, instantiate SFB12 (BSEND): assign ID and R_ID = DW#16#10.
- In the CPU 414-5H, instantiate SFB13 (BRCV): assign matching ID and R_ID = DW#16#10.
Compile and Download Network Connections:
- In NetPro, select Network -> Save and Compile -> Compile completely.
- Select the CPU 410-5H station -> select PLC -> Download to Current Station -> Connections and Gateways.
- Select the CPU 414-5H station -> execute identical download of connections.

Verification

Verify fault-tolerant communication stability through the following checks:
1. In NetPro online mode (PLC -> Open Online Partner): verify that the S7 fault-tolerant connection displays status Established.
2. In CFC or LAD test mode, monitor BSEND.DONE and BRCV.NDR: confirm that new data pulses high cyclically with ERROR = FALSE.
3. Perform a redundancy switchover on CPU 410-5H: confirm that data transmission continues uninterrupted across the standby controller.

Limitations and Operational Precautions

Downloading connection tables (Connections and Gateways) to an S7-400H controller pair can be performed in RUN mode provided the station configuration was not drastically altered. However, never execute a 'Download Station' or 'Entire Hardware Config' without verifying that the partner CPU is synchronized in redundant mode; downloading uncompiled connection lists can force the controller into STOP. Always ensure that R_ID values are unique across all communication blocks on the same connection.