Configuring a PROFINET Ring Topology with SCALANCE X-400 Switches and an S7-400H System

This reference describes how to engineer a redundant PROFINET ring built from three SCALANCE X408-2 switches, an S7-400H fault-tolerant controller, and an S7-300 station in SIMATIC Manager. The ring is closed through the third SCALANCE, redundancy is provided by Media Redundancy Protocol (MRP), and the two PROFINET interfaces of the S7-400H CPU are both attached to the same Ethernet/PROFINET subnet so that the H system can continue to communicate if one ring segment or one switch is lost.

All configuration steps use STEP 7 V5.5 + SPx with the SIMATIC NET configuration tools and the SCALANCE X-400 GSD package installed in HW Config. STEP 7 V13 / TIA Portal variants are not covered here; for those workflows use the SIMATIC PROFINET system description referenced below.

1. Topology Overview

The physical layout below shows a closed ring on port pairs 1 and 2 of every SCALANCE X408-2. The PROFINET interface of CPU 0 (rack 0) of the S7-400H is wired to SCALANCE 1, the PROFINET interface of CPU 1 (rack 1) of the S7-400H is wired to SCALANCE 2, and the S7-300 IM 151 / CP 343-1 is wired to SCALANCE 3. The fiber connections between switches are realized through two MM491-2 media modules per switch (one for the upstream, one for the downstream ring segment), so that the entire ring is multimode fiber 50/125 µm with SC connectors.

The blue lines are the primary active path (data + ring closure), the orange dashed lines are the physical connections from the controllers and the S7-300 into the ring. In normal operation the MRP manager blocks one of its ring ports so the topology behaves as a logical line; on a fault the manager unblocks the port and the ring is re-established within the recovery time of the MRP manager.

2. Prerequisites and Component List

Reference: SCALANCE X-400 Operating Instructions (Siemens Industry Online Support), SIMATIC PROFINET System Description.

3. MRP Fundamentals and Ring Port Selection

MRP (IEC 62439-2) defines three roles that coexist in a single ring:

For an S7-400H plant with an S7-300 attached, the recommended choice is:

• SCALANCE #1 = MRP Manager (fixed role).
• SCALANCE #2 and SCALANCE #3 = MRP Client.

For an 8-switch ring the standard IEC 62439-2 recovery time is 200 ms (default; the manager sets it via test frame); the SCALANCE X-400 default configuration is 200 ms and is suitable for most H systems. STEP 7 also offers a Prioritized startup option for the S7-400H where the H CPUs bring the ring up faster than other devices; this is configured in HW Config → Properties of the S7-400H PROFINET interface → Prioritized startup.

Reference: SIMATIC PROFINET System Description — Media Redundancy, SCALANCE X-400 Web Based Management (WBM) Manual.

4. Creating the S7-400H Station and a Shared PROFINET Network

The root cause of the most common configuration error in this topology is the duplication of the Ethernet subnet: STEP 7 will create a new Ethernet/PROFINET subnet for every CPU that has its PROFINET interface inserted, and if the H system is built by adding a second CPU afterwards, the second PROFINET interface is automatically attached to a brand-new subnet. The two H CPUs then end up on different subnets and the S7-300 cannot reach both, which is exactly the symptom reported in the field report ("both CPUs had the same network this time" — that is the success criterion).

1. Open the SIMATIC Manager and create a new project.
2. Insert a SIMATIC 400H Station from the right-mouse menu of the project. The HW Config of the H station opens.
3. Insert the UR2-H or UR2 ALU rack (e.g. 6ES7 400-1TA01-0AA0) and place the PS 405 power supplies in slots 1 and 3 (Rack 0) and Rack 1 of the H system.
4. Insert CPU 0 (e.g. 6ES7 414-4HM14-0AB0) into slot 3 of Rack 0. The CPU carries the PROFINET interface (X1 / X2). The slot for the CPU of Rack 1 is a slot that mirrors the same slot number (e.g. slot 3 of Rack 1) — STEP 7 will not place the second CPU in the same physical slot of the second rack; you must manually insert it.
5. When the PROFINET interface of CPU 0 is inserted, STEP 7 displays the dialog Properties — Ethernet Interface. Click New… and create a PROFINET subnet. Configure:
   • Subnet name: PN-MRP-Ring (descriptive name)
   • IP address: e.g. 192.168.0.10
   • Subnet mask: 255.255.255.0
   • Router: leave empty unless routed networks are required
6. Insert the CPU 1 (6ES7 414-4HM14-0AB0) into the matching slot of Rack 1. When its PROFINET interface is inserted, STEP 7 will again offer a subnet — do not create a new one; instead select the existing PN-MRP-Ring subnet. This is the critical step that binds both H CPUs to the same PROFINET subnet.
7. Assign CPU 1 a unique IP address (e.g. 192.168.0.11). Both CPUs must be on the same subnet mask, otherwise the S7-300 cannot communicate with both.

Reference: S7-400H System Manual — H Station configuration.

5. Adding the Three SCALANCE X-400 Switches to the PROFINET Subnet

From this point on, all work continues in HW Config of the S7-400H station, not in NetPro. NetPro is only used for cross-referencing networks; the PROFINET IO topology is owned by HW Config.

1. In HW Config, open the PROFINET IO catalog (right side panel). Under PROFINET IO → Network Components → SCALANCE X-400 the installed GSD exposes the SCALANCE X408-2 module.
2. Drag the SCALANCE X408-2 onto the PROFINET network line of the H station. STEP 7 prompts for an IP address; assign:
   • SCALANCE #1: 192.168.0.1
   • SCALANCE #2: 192.168.0.2
   • SCALANCE #3: 192.168.0.3
3. Open the Properties dialog of each switch and confirm that the Device name is unique and matches the labelling in the cabinet (e.g. x400-1, x400-2, x400-3). The device name is the address used by PROFINET IO discovery; it must be lower case, no spaces, max. 240 characters, must start with a letter.
4. Confirm that STEP 7 places every SCALANCE on the same PN-MRP-Ring subnet. The IO controller of this subnet is the S7-400H; the S7-300 is an IO device (or just a PN node, depending on what is wired to it).

6. Configuring MRP Roles and Ring Ports

The MRP configuration in STEP 7 is not done in the normal PROFINET interface properties; it is done in the PROFINET IO Domain Management dialog. This is a frequent point of confusion because the ring is invisible in the graphical network view.

1. Right-click the PROFINET IO line in HW Config and select PROFINET IO Domain Management.
2. The Domain Management dialog lists every PROFINET device in the project, its current MRP role, and the ring ports that will be used. Assign:

   Device	MRP role	Ring port 1 (P1)	Ring port 2 (P2)
   SCALANCE #1	Manager	MM491-2 — Port 1 (FX1)	MM491-2 — Port 2 (FX2)
   SCALANCE #2	Client	MM491-2 — Port 1 (FX1)	MM491-2 — Port 2 (FX2)
   SCALANCE #3	Client	MM491-2 — Port 1 (FX1)	MM491-2 — Port 2 (FX2)

3. Verify the MRP domain identifier is identical for all three devices; STEP 7 names it MRP-Domain_1 by default and it is the only domain an MRP manager can supervise.
4. Select the Prioritized startup check-box for the S7-400H PROFINET interface if the S7-300 IO device should reach the H CPU faster after a ring break (typical when an S7-300 IO device must be served within 300 ms of a recovery).
5. For the SCALANCE X408-2 you can also set the MRP role in the Web Based Management under Layer 2 → MRP, but the project is the source of truth — keep WBM consistent with the STEP 7 project.

Reference: SCALANCE X-400 WBM — MRP configuration.

7. Integrating the S7-300 Station on the Ring

The S7-300 can be attached to the ring in two ways. The recommended approach is a dedicated PROFINET IO device relationship with the S7-400H acting as IO controller.

1. Insert a SIMATIC 300 Station into the project.
2. Open HW Config of the S7-300, insert the rack, the PS 307, the CPU 31x PN/DP and a CP 343-1 Lean if the CPU is not a PN type.
3. Select the PROFINET interface of the CPU (or CP) and choose the existing PN-MRP-Ring subnet — do not create a new subnet.
4. Assign an IP address (e.g. 192.168.0.20) and a PROFINET device name (e.g. s7300-cp).
5. In the S7-400H HW Config drag the S7-300 PROFINET interface into the PROFINET IO controller slot of the H CPU. STEP 7 will create the IO device relationship and assign a slot number for the S7-300 (slot 0 of the IO device is the head module / CPU interface).
6. Run Save and Compile; the result must show the S7-400H as IO Controller and the S7-300 + 3 SCALANCE devices as IO Devices in the same domain.

Reference: SIMATIC PROFINET System Description — PROFINET IO, S7-400H — PROFINET IO with redundant interface.

8. PROFINET IO Domain Management and Compilation

After all three SCALANCE devices and the S7-300 are inserted, compile the S7-400H station. The compiler must report:

• No errors and no warnings on the PROFINET subnet.
• All IO devices show the same IO system number (e.g. PROFINET IO System (100)).
• The S7-400H appears as IO Controller in two roles (one per CPU), and the IO system is the same on both PROFINET interfaces.

If a device shows the warning "Device is not part of any IO system", it is usually because the device was inserted by drag-and-drop from the catalog and the wrong IO controller was selected at insertion time. Delete the device, drag it from the catalog again, and drop it directly on the PROFINET line of the S7-400H.

9. Downloading the Configuration and Verification

1. Connect the engineering PG to one of the SCALANCE X-400 ports (any RJ45).
2. Set the PG/PC interface to TCP/IP → Intel Ethernet with a static IP in the same subnet (e.g. 192.168.0.100).
3. Open PLC → Download to Target System → Selected CPU in HW Config of the S7-400H. The downloader will discover both H CPUs on the subnet; download the project to both.
4. Download the S7-300 station (CPU 31x PN/DP) in the same way.
5. Trigger a Topology Discovery via the WBM of SCALANCE #1 to confirm the ring is closed (Layer 2 → Topology → Accept). The expected result is a single MRP ring with three members.
6. Open the Online & Diagnostics view of the S7-400H PROFINET interface. The IO Devices tab must show the three SCALANCE and the S7-300 as OK.
7. Run a Ring test: pull one of the SCALANCE SFP/Glass fiber pairs. The MRP manager must re-establish the ring within 200 ms and no PROFINET IO device should go to Fail. The H system should not switch.

10. Troubleshooting Matrix

Reference: SCALANCE X-400 Operating Instructions — Diagnostics, SCALANCE X-400 WBM Manual.

11. Performance, Timing, and Safety Considerations

• MRP recovery time on a 3-switch ring is ≤ 200 ms; with up to 50 MRP nodes the IEC 62439-2 default is 200 ms. The H system tolerates a 200 ms ring break without switching.
• PROFINET update time of the S7-300 IO device should be set to ≥ 1 ms. With three SCALANCE devices in the ring, the minimum is 250 µs; for IO devices that are not time-critical, 1 ms is the recommended starting point.
• CPU 414-4H vs. CPU 417-4H: the 417-4H supports more PROFINET IO devices and a higher PROFINET update rate; the 414-4H is sufficient for a 3-switch ring with one S7-300 IO device.
• Fiber segments with MM491-2: 50/125 µm multimode, 0–3000 m at 100 Mbps full duplex. Do not mix with single-mode MM491-1LD media modules on the same ring.
• Redundancy vs. parallel operation: with PROFINET IO on the H system, both PROFINET interfaces of the H system are on the same subnet (this configuration). For the H system to keep IO communication after losing one CPU's PROFINET interface, configure PROFINET IO System 100 on the first CPU and PROFINET IO System 101 on the second CPU and assign the S7-300 to both IO systems; this is a more advanced setup outside the scope of the basic MRP ring.

Reference: S7-400H System Manual — H PROFINET IO redundancy, SIMATIC PROFINET System Description — Update time.

12. Operational Notes and Field-Proven Caveats

1. When expanding the ring (adding a fourth or fifth SCALANCE), insert the new device on the same PN-MRP-Ring subnet. STEP 7 will then offer the existing PROFINET IO Domain Management; add the new device as Client and assign its ring ports. Re-compile and re-download to all CPUs.
2. If the SCALANCE X408-2 is used with an S7-400H, the S7-400H should always be the IO controller of the subnet. The S7-300 is best configured as an IO device, not as a stand-alone PN node, so that the H system can supervise it.
3. When commissioning, always check the MRP state via WBM before opening any PROFINET connection. The Information → MRP page shows the active manager, the recovery time and the ring port status.
4. Always store the STEP 7 project on the engineering server and the as-built cabinet drawings should include the device name and the ring port label (e.g. x400-1 — ring ports: MM491-2/P1, MM491-2/P2).
5. For an H system that has a SIMATIC PCS 7 operator station on the same ring, configure the ring with MRP and additionally enable the Passive listener role on the operator station switch (or use a dedicated PC switch, not part of the ring, to keep the ring pure).

FAQ