S7-400H Redundant Ethernet Network Design with CP443-1, SCALANCE X Rings, and RedConnect

This technical reference describes how to design a fault-tolerant Ethernet plant network for multiple SIMATIC S7-400H high-availability controllers, supervisory IPCs (HMI stations), and a higher-level Modbus system. The architecture presented is based on dual CP443-1 Ethernet couplers per H-CPU, a SCALANCE X204-2 fiber ring, CP1613 A2 modules on the IPCs running the S7-REDCONNECT software, and a dedicated Modbus gateway path. Every part number, firmware assumption, and cabling rule stated here is restricted to what is documented for the referenced Siemens manuals.

1. Network Architecture Overview

The reference topology uses a single plant Ethernet ring that interconnects the four S7-400H systems and the four IPCs. Each H-system contributes two CP443-1 modules, one attached to ring segment A and one to ring segment B. The IPCs connect through CP1613 A2 PCIe cards that run the S7-REDCONNECT redundancy software so that the HMI can address the active CPU of each H-pair transparently.

The ring is built from SCALANCE X204-2 managed switches. The X204-2 is a four-port switch with two of those ports being optical (100 Mbps, SC sockets) and the remaining two being electrical. The optical pair is what closes the ring via multimode fiber. The electrical ports drop down to the CP443-1 modules and the CP1613 A2 cards in copper.

2. CP443-1 Module Selection and H-CPU Coupling

Each S7-400H pair must terminate the plant Ethernet on two distinct CP443-1 modules so that a single CP failure does not disconnect the controller from the ring. The 6GK7443-1EX20-0XE0 is the classic CP443-1 for H-system operation; it supports ISO-on-TCP, TCP, UDP, S7 communication, and the S7-REDCONNECT redundancy protocol that lets a single partner address either CPU of the H-pair.

Each CP443-1 is plugged into the backplane of its own S7-400H sub-rack (rack 0 for CPU 0 and rack 1 for CPU 1 in the H-station). The CPs are assigned different IP addresses within the same subnet, for example 192.168.10.11 and 192.168.10.12 for H-system #1. The H-system itself exposes a single redundancy IP address (e.g. 192.168.10.10) that the HMI uses; S7-REDCONNECT resolves this virtual address to the currently active CPU.

2.1 STEP 7 / HW Config layout per H-system

1. Open the S7-400H station in HW Config (STEP 7 V5.5 or TIA Portal with the S7-400H add-on).
2. Insert two CP443-1 (6GK7443-1EX20-0XE0) modules, one in slot 4 of rack 0 and one in slot 4 of rack 1.
3. Open the properties of each CP and set the MAC address, IP address, and subnet mask. The redundancy IP address is configured once on the H-station object.
4. Enable "S7-REDCONNECT" or "H-CPU connection" depending on firmware version. For firmware V6.x and newer, this is labeled "H connection" in the connection list.
5. Compile and download to both CPUs.

3. SCALANCE X204-2 Ring Topology and HRP/MRP Setup

The SCALANCE X204-2 (6GK5204-2BB10-2AA3) is a managed Industrial Ethernet switch with two 100 Mbps multimode fiber ports (SC) and two 10/100 Mbps RJ-45 ports. The fiber pair is used to form a High-speed Redundancy Protocol (HRP) ring, which is Siemens' ring protocol implemented in parallel with the standardized Media Redundancy Protocol (MRP). HRP and MRP are not interoperable; if the ring is purely SCALANCE, enable HRP; if non-Siemens MRP nodes are present, use MRP instead. The decision path matters because ring nodes that do not speak the same protocol will leave the ring unclosed.

3.1 Ring port assignment

3.2 HRP configuration steps in Web Based Management (WBM)

1. Open the WBM at the management IP of the X204-2 (e.g. https://192.168.10.101).
2. Navigate to Layer 2 > Ring and select "HRP Manager" on one switch, "HRP Client" on the others.
3. Assign the two ring ports (Port 3 and Port 4) to the ring.
4. Set the HRP observation time. Default 200 ms is acceptable for 100 Mbps multimode rings with up to ~50 nodes. For longer ring reconfiguration, raise to 500 ms.
5. Save and reboot. The ring closes automatically once the manager sees its two ring ports as a single segment.

With six SCALANCE X204-2 nodes the maximum fiber length between two adjacent nodes is dictated by the 100 BASE-FX budget, typically 5 km with 50/125 µm multimode and 4 km with 62.5/125 µm. For a plant-wide ring, plan at least 6 dB of optical power margin per segment to allow for aging and connector loss.

4. IPC Connection via CP1613 A2 and S7-REDCONNECT

An IPC that supervises an S7-400H pair cannot simply use the onboard Ethernet port, because plain TCP/IP does not know which of the two CPUs is currently active. The CP1613 A2 (6GK1161-3AA01) is a dedicated PROFINET/Industrial Ethernet PCIe card that runs the S7-REDCONNECT software on the IPC. With RedConnect, the IPC opens one logical connection to the H-system's redundancy IP address; the runtime then fails the connection over to the standby CPU within roughly 100–300 ms if the active CPU fails.

4.1 IPC prerequisites

• Windows 7 / Windows 10 / Windows Server (release per WinCC version, see WinCC release notes).
• SIMATIC NET PC software, including S7-REDCONNECT, version aligned with the CP1613 A2 firmware.
• WinCC (TIA or classic) configured with four tag groups, one per S7-400H package.
• CP1613 A2 firmware compatible with the SIMATIC NET version installed.

4.2 S7-REDCONNECT connection table (per H-system)

When WinCC is then configured to use "S7-REDCONNECT" as the channel, four tag groups can be created - one per H-system - and the IPC can monitor any H-system from any IPC because all four HMI stations share the same ring and the same IP routing. WinCC treats each H-system exactly as a normal S7 target; the redundancy is handled transparently below the channel DLL.

5. PROFINET IO on S7-400H with IM 155-5 PN HF

Although the S7-400H CPUs themselves have integrated PROFINET interfaces on selected variants, distributed I/O is typically added through PROFINET interface modules. The IM 155-5 PN HF (6ES7155-5AA00-0AC0) is documented as supporting system redundancy on S7-400H, meaning that two IM 155-5 PN HF modules can be operated as a redundant PROFINET pair on the H-station. This is the official path for adding PROFINET IO to an S7-400H controller with failover behaviour.

Per the Siemens documentation on system redundancy, the IM 155-5 PN HF can be operated on the PROFINET IO of an S7-400H. Two such IMs are typically configured as a redundant pair on the H-CPU side: one bound to CPU 0 and one bound to CPU 1. The PROFINET devices (ET 200MP, ET 200SP, drives, switches) are addressed through whichever IM is currently the active interface for the IO line.

6. Modbus Uplink Options

The project also requires a redundant Modbus uplink to a higher-level control station. The S7-400H system itself does not deliver native Modbus redundancy in the controller's firmware - the redundancy must be built at the CP level. There are two distinct paths.

6.1 Modbus RTU (serial) on CP341 / CP441-2

The CP341 (6ES7341-1CH02-0AE0) and CP441-2 (6ES7441-2AA05-0AE0) point-to-point modules can be loaded with the Modbus Master or Modbus Slave loadable drivers. The Modbus link is serial (RS-485 or RS-232) and is therefore inherently non-redundant at the physical layer. To obtain a redundant Modbus RTU uplink, install two CP341 / CP441-2 modules in the H-station (one in each sub-rack) and connect them to two independent Modbus lines. The application program on the H-CPU selects which channel to read from / write to, or reads both and selects the fresher value.

6.2 Modbus TCP on CP443-1 (Modbus gateway)

The CP443-1 (6GK7443-1EX30-0XE0) acts as a Modbus TCP gateway for an S7-400 / S7-400H station. With one CP443-1, a single link exists. The CP443-1 EX30 has documented support for Modbus TCP server functionality. For a fully redundant Modbus TCP path, deploy two CP443-1 modules in the H-station on the same Modbus TCP subnet and let the higher-level control station connect to the redundancy IP. Whether this is fully transparent at the Modbus level depends on the version of the Modbus TCP loadable driver and the firmware of the CP443-1. A pre-release product was announced for redundant S7-H communication via Modbus TCP using the CP443-1; verify availability and firmware prerequisites against the current Siemens product catalog before committing the design to this path.

The selection criterion is unambiguous when read against the source: Modbus RTU is supported today as a non-redundant serial link; Modbus TCP redundancy on the CP443-1 has been a product roadmap item and must be re-validated for the actual firmware level in the project.

7. HMI Configuration with WinCC and Cross-IPC Monitoring

The requirement to monitor every S7-400H package from every IPC is satisfied naturally once all IPCs and all H-systems share the same ring and the same subnet. Each IPC maintains four S7-REDCONNECT connections, one to each H-system's redundancy IP. WinCC tags are organized into four tag groups (one per H-system) so that screen navigation can group variables by package.

7.1 WinCC channel assignment

1. Add the SIMATIC S7 PROTOCOL SUITE channel and the S7-REDCONNECT sub-channel.
2. Create four logical connections named HMI_H1 through HMI_H4, each pointing to one H-system's redundancy IP (192.168.10.10 / .20 / .30 / .40).
3. Define four tag groups Package_1 through Package_4. Tags are placed in the group that corresponds to the H-system they belong to.
4. On every IPC repeat the same four-connection setup. Because all IPCs share the ring, they can address every H-system.
5. Enable the "Connection status" system tag so the operator can see which H-system is currently the master for the active connection.

7.2 Cycle and update time

For a typical WinCC runtime, the S7 update time for a connection is in the 250–500 ms range. The H-CPU failover is at most 100–300 ms with RedConnect, so the operator may observe a single frozen update cycle at the moment of failover. Tune the WinCC acquisition cycle to 500 ms to avoid log file gaps during a switchover.

8. Network IP Plan and VLAN Design

Use a dedicated subnet for the plant Ethernet ring and keep the HMI network isolated from the field PROFINET and the Modbus TCP network. A three-VLAN design is recommended.

Configure the SCALANCE X204-2 as the Layer-2 boundary between VLAN 10 and the other two VLANs. The X204-2 itself does not route; pair it with a SCALANCE XR-100 or X-300 series if Layer-3 routing is required between VLANs.

9. Commissioning Procedure

1. Power up the SCALANCE ring. Verify the HRP manager logs a closed ring (event "Ring closed" in WBM).
2. Bring up the S7-400H sub-racks one at a time. After each sub-rack, verify that the CP443-1 reports LINK and that it is reachable via PING from a service laptop.
3. Download the S7-400H project to both CPUs. Verify the H-station is in RUN-redundancy and that both CPUs show "H-Sync OK" on the HMI panel of the CPU.
4. Install the CP1613 A2 in each IPC. Install the matching SIMATIC NET PC software and the S7-REDCONNECT license.
5. Configure the four S7-REDCONNECT connections in WinCC. Verify that the connection status turns green and that test tags toggle with the expected value.
6. Configure the Modbus path (RTU via CP341 / CP441-2 or TCP via CP443-1 EX30). Verify with a Modbus scanner on the higher-level station.
7. Cut one fiber in the SCALANCE ring. The HRP manager should report "Ring open" within 200–500 ms; all connections must remain green.
8. Pull CPU 0 from the H-station. The redundancy should failover to CPU 1; all WinCC tags must continue to update with a maximum gap equal to the configured acquisition cycle.
9. Pull one CP443-1. The H-system must continue to be reachable via the surviving CP443-1. The S7-REDCONNECT connection must remain green.

10. Troubleshooting Matrix

11. Field-Proven Caveats and Engineering Rules

• Two CPs, not one. The S7-400H H-station provides slot 4 in each sub-rack for a CP443-1. Use both. A single CP443-1 is not a redundant path even if it is on a redundant ring.
• Redundancy IP, not CPU IP. The HMI and any other Ethernet partner must always target the H-station's redundancy IP. Targeting a specific CPU IP defeats the failover.
• HRP manager uniqueness. Exactly one SCALANCE in the ring must be the HRP manager. Two managers (or none) cause the ring to flap.
• Fiber budget. The 100 BASE-FX ports of the X204-2 deliver roughly -19 dBm transmit power at 1310 nm. Plan a 6 dB aging margin across the full ring path.
• VLAN boundary. The X204-2 does not route. Do not place the PROFINET and Modbus subnets inside VLAN 10 unless a Layer-3 device is in the path.
• CP1613 A2 driver alignment. The CP1613 A2 firmware and the SIMATIC NET version on the IPC must match. A mismatch shows up as the channel staying red even though the S7-400H is healthy.
• Modbus TCP redundancy. Confirm with the current Siemens product catalog whether the Modbus TCP gateway of the CP443-1 (6GK7443-1EX30-0XE0) supports a redundant configuration in your firmware level. Treat it as a product roadmap item until confirmed.

12. Summary of Hardware and Part Numbers